I've been playing Minecraft since it came out, and I started using mods once we started to see some of the first major ones, such as pixelmon. I've played less over time, but I still throw it on to chill out every once in a blue moon. Now, here's my issue. Why are almost all mods either for 1.7 or the most recent version with no in-between? I love the older versions, and I'll admit that there's a charm to the newer updates, but 1.9.4 is where I'm happiest. It's still the og Minecraft, just with some fun expansions to give you things to do after the End. I don't want to give up a lot of the cool combat and adventuring features to go back to 1.7, but I just can't get into the chaos of the newer updates. Is there some reason I'm not aware of that explains why no one makes mods for 1.9? Did something change? Or is it just that 1.7 was the actual cut-off for "classic minecraft". I'm missing something here, enlighten me if you can.
(I'm talking mods like advanced energistics, twilight forest, etc)
Not sure why the versions prior to 1.7.2 aren't there, I think many of them were on the MCForums prior to Curseforge at a guess. Even a quick look here: http://modlist.mcf.li/version shows many of the mods for prior versions and many lead to the MCForums.
I've found many from modpacks from 1.2.5 or 1.4.7/1.5.2 when I want many of them or mod repost sites (not recommended but many of them are there even ones I'd never heard of). Otherwise there is communities that focus on older versions back in alpha/beta or around 1.0. Otherwise I've just found many mods through the wayback machine/archive.org besides modpacks from certain versions back.
If you look at 'date created' you can find many for 1.9 still being made, not as many as 1.14/1.15 as they are new versions, 1.12.2 and 1.7.10 are popular for their own reasons (1.12.2 is the newer 1.7.10 of popular versions but 1.7.10 has it's fans and mods). I make videos on mods from 1.9 that are from their relevancy and new so I check every so often to see what comes up. https://www.curseforge.com/minecraft/mc-mods?filter-game-version=1738749986:552&filter-sort=1
1.7.10 was just popular due to changing the biomes (I assume) but for modding I don't know. The 1.9 update did bug people and 1.8 didn't offer much I assume for Vanilla or Modding but 1.8.9 is good for PVP if you hate the 1.9 combat features. 1.7.10 now is just a cut off point due to the awkward changes Forge went through in 1.8 that made devs on 1.7.10 either give up Minecraft or stay on 1.7.10 (especially if their mod isn't suitable to update because a lot of work would need to be done like RotaryCraft for example). Otherwise 1.7.10 is just suitable for some people to play the game if they don't like the newer versions features or if they want to run their devices without having to upgrade their hardware but still have fun.
Minecraft requiring 4GB to play Vanilla is a bit much (check the specs, it has gotten that big the game over time), you don't need that much but for recommended hardware and for Forge 1.14 it is needed. Back in 1.7.10 with a decent PC you can run the game well and with mods, in 1.10.2+ you'd be looking at 2GB+ (I couldn't run my old PC with a few mods in 1.10.2 so I had to be careful what I used) just for it to work well, and 1.13+ made you need more. So 1.7.10 is just suitable for people for what hardware they have and mods still being made for it big and small.
Big mods like Twilight Forest (not sure about Advanced Energistics (I assume you mean Applied Energistics) have come to 1.10.2 and 1.12.2 (I've used them a few times and otherwise done retrospectives on them through their start to their latest versions so I know the states they are in currently [source of my retrospectives: https://www.youtube.com/playlist?list=PLbOPi5L7V78J-35pncbHr4CQa1QzoFzlz]).
The reason why 1.7.10 is so popular is because there were major changes to rendering and how block metadata works in 1.8 (mainly), which made it difficult to update many mods, especially ones which use custom rendering methods (IIRC one modder said they'd need to make thousands of model json files for a single block, or resort to using tile entity rendering and all of its downsides, like performance). 1.8 also greatly increased the resource usage of the game due to poor coding decisions on Mojang's part (mainly the aforementioned models, as well as things like using an object instead of x,y,z variables to handle coordinates everywhere).
1.12.2 is pretty similar, again mainly because of large-scale code refactoring in 1.13 (the resource usage of the game is simply crazy when compared to 1.6.4, which I still play on and develop my own mods for in part due to this (even 1.7.x had issues on my old computer, and I still see "client side chunk ticking" causing a lot of lag, worse than 1.8 actually, which I presume is why some people say that 1.8 improved performance for them); I only allocate 512 MB of memory of which only about half is actually even allocated, much less used, and I never experience server lag (after fixing MC-17630 myself) - in fact, I've reduced overall resource usage despite adding many vanilla versions worth of new features - it is often said that more content is why the game has gotten more demanding but it is simply down to bad code. Even 3rd-party tools like AMIDST are much slower due to using code from the game - that slower biome generation is absolutely not because they added a few new ocean biomes - a developmental version of TMCW adds over 100 new biomes to 1.6.4 with a much more complex biome generator, yet it is actually faster and uses less memory).
The situation hasn't improved much since then; Optifine, usually one of the first mods to update to a new version, still hasn't updated to 1.15 (I have no idea how the new 1.13+ mod loaders, like Rift and Fabric, can be updated so fast as to be available for snapshots but I presume they are very basic).