I'd say the minecraft wiki is the most likely one to point towards, but not a '1.7.10 - 1.12.2' complete block/item/tweak comparison like I'd say exists similar to say a converter like presentation (was in my head at the time so excuse that part). Otherwise either Mojang it's might, or you could yes download each version have take a look to find out in creative for blocks besides the odd menu stuff like 1.8's customised world gen and other versions like 1.12 with it's narrator/recipe book features that I assume no one uses besides a small group of people even though I myself liked 1.12 personally.
Many mods for 1.7.10 do focus on 1.8 features to 1.7.10 for example, others for later versions add stuff available backwards, more so in the 1.9/1.10.2 time period I remember seeing some for, like for elytra benefits.
Depends on the features from each version. Like 1.8 blocks for underwater monuments and a bunch of of block related stuff mostly, make senses for some 1.7.10 to want them, hence a mod literally for that I know of :), 1.9 not so much I'd assume for many people but the shields, combat, extra End region with the End City/Shulkers, Elytra as gliders, 1.10 adding very little, 1.11 also very little, 1.12 being big with terracotta/recipe book in the inventory for crafting tables (not furnaces yet), a narrator for chat/commands and the odd block like content things. That about sums up each update
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'Niche Community Content Finder' for fun and support of creators. I look for mods that interest me or need a bit of acknowledgement or bug fix testing/reporting. I also make modpacks/maps
Creator/Maintainer of Fabric, Rift & Tesla Mod Discussion Threads/Niche Mod Youtuber.
As SuntannedDuck2 says it won't let you compare two random versions directly but the page for each version has a list of changes.
So, in your example, you would have to copy the lists for 1.8, 1.9, 1.10, 1.11 and 1.12, then go through and cross out any changes that have been reversed in a later version and what's left should be a list of the differences between 1.7 and 1.12.
It might take some additional rearranging and formatting to get say all the added blocks together in one part of the list and changes to mobs in another, etc but it sounds close to what, I think, you're asking for.
The wiki may be a good starting point, but it is far from an exhaustive list of changes; nor do the official descriptions of each release seem to include every change…
Technical changes in particular appear to get short shrift…
[Recent changes to the length of time chunks in a dimension stay loaded after a nether portal is used, and the way loaded chunks are prioritized for unloading (to neame two recent examples) have both broken many previous techniques/devices.]
For the goal of modding 1.7 to resemble current versions, I would think it best to identify which features you wish to mod in and focus on those areas.
Comments and discussions about builds broken by the 'next' version may be useful in identifying unannounced changes that may be necessary for the new 'thing' but break something you'ld like to retain.
You may also be able to identify areas you can ignore (ie. unless you want to add shields, none of the changes to the original implementation are likely to be important; conversely, if you do wish to add them you need be concerned only with the version you want).
Should you be fortunate enough to find a site/source that allows the sort of comparison you wish to make, please do pass that resource on….
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.
It isn't really necessary to perfectly replicate newer versions; you can tweak features to better fit the way that you want them to work; for example, when I added the 1.8 stones to TMCW at first I had them work like the vanilla versions when you mine them but soon I made my own patch which made Silk Touch be required to mine the raw variants, otherwise dropping cobblestone, before making it a permanent feature, so they wouldn't get in the way when mining. Likewise, I removed brewing stands from igloos (fun fact: they made them require blaze powder for fuel so as to not break game progression by adding them to igloos).
I also made podzol craftable with 2 dirt and 2 spruce leaves, making 2 podzol (vanilla later made it "craftable" by making large spruces turn grass under them into podzol). I added Mending as a direct replacement for renaming an item to keep the cost from increasing (instead of repairing it with XP you pick up; combined with the pre-1.8 anvil repair mechanics I think this is much more balanced in terms of how many enchantments you can put on gear).
More recently, I added husks and strays, which replace 50% of zombies anywhere in hot/snowy biomes and a few others (instead of 80% only at the surface), with more variants to match their biomes (e.g. red in mesa and pink in the Nether), same for endermites (which also spawn when an enderman is attacked by the player, which was originally in vanilla but later removed; the Nether also has a pink "nethermite" variant).
Some other features have been added more for fun, such as adding 14 new variants of walls (similar to 1.14 but with different block types), based on the vanilla block (which uses 2 out of 16 data values, so I didn't actually add new blocks; likewise, I added variants for fences by using metadata instead of new block IDs, partly to show how Mojang could have conserved block IDs. Another block, of my own design, that I added has over 100 variants in a single block without using a tile entity, which I added to flowerpots (as 1.7 did) so they can render over 60 different plants, and many more sub-variants, even miniature full-grown trees).
I'd also look at re-adding older features; I made giants naturally spawn and updated them to the new AI so they can properly pathfind (1.8 removed their AI altogether), with their damage more balanced (10 instead of 50). Similar to skeletons, creepers used to be able to move while counting down but they lost this around 1.2 and it hasn't been re-added yet (to offset this, as well as a reduction in player armor protection from 80% to 66% (unlike 1.9 armor and weapon nerfs only affect the player), I reduced their peak damage from 49 to 36 and reduced the rate the damage deceases with distance, making them deal more damage from further away). While not a thing in any vanilla version I made cave spiders naturally spawn underground, fitting their name, and naturally spawning silverfish (they can't enter blocks, only ones spawned from spawners or monster eggs), increasing the variety of mobs that the player may encounter on a regular basis without adding any new ones.
The underground in 1.7+ could also seriously benefit from a reversion to the 1.6.4 generation (or better; I have a mod for 1.7.10 that can be used with most Forge mods (confirmed by somebody who uses it with a big modpack) that adds the cave generation in TMCW, if not its other underground features). 1.7 could also really benefit from changes to the climate system that makes the same few biomes generate over large areas (for example, this is a world I played on using TMCW; there are also many of my own biomes).
Note that from that I've seen it is extremely difficult to mod vanilla code with Forge, one reason why I never learned how to use it; either that, or mods add their own versions of the vanilla blocks/items they would otherwise modify (for example, a slime block mod for 1.6.4 adds its own pistons instead of modifying vanilla pistons); this can have some other undesired effects, such as adding the 1.8 stone variants using new block IDs (vanilla ores only replace stone so this could significantly impact ore frequency unless you add your own generator). It also significantly increases the resource requirements due to how it integrates mods into the game (I've kept memory usage on the order of 100 MB despite adding 300+ features in an upcoming update alone, and it takes a couple seconds to launch the game). Of course, if you want mod compatibility you need Forge.