I have several questions about the concept of suggestions on the Forum.
First, how does anyone here expect there suggestions to be added to the game? Is it a hope that the suggestion will gain a lot of traction in the community? Or do actual Mojang employees visit the Forum?
Second, I would like to know some notable suggestions that have been added to the game in some form, if any.
Finally, are there any notable suggestions that have not been added to the game, but are immensely popular and have a lot of traction going for them?
To the best of my understanding, this isn't really an 'official' forum in that it's more of a place to discuss ideas than anything. Mojang does have an official feedback website that I presume is more closely followed by the developers.
By this metric, the most popular suggestion is "Cubic Chunks", which originated as a mod that split chunks into 16x16x16 pieces, and was actually partially implemented in 1.2 as the "Anvil" file format (source), but it only internally handles chunks in 16x16x16 pieces, they are still generated and loaded as a single unit spanning the height limit (however, empty sections are not saved or loaded so this does reduce resource usage).
The next most popular suggestion is colored light and related ideas, some of which have been implemented, such as colored glass and beacon beams, although light itself has not been changed and would require significant internal changes (possibly not as significant as you might think; rendeirng already supports colored light in the form of applying a color multiplier to a block's base color, e.g. shading leaves according to the biome; and this, as well as smooth lighting calculations, involves handling RGB color components separately. the main issue is that you'd need 3 times the data storage and computations for lighting).
After this is the "Aether", which itself is based on the original "Sky Dimension", as the End was going to be before it was changed to its current form, which is a lot like the "floating islands" world type, and is by far the oldest of the three, posted back in late 2010.
Fourth, but not least, is a Nether update, which actually occurred in 1.16, if not in the same way as the suggestion, but updates like this have been popular (along with such ideas as "cave update", which was especially popular after 1.7 nerfed them; funnily enough, before then there were more threads complaining that there were too many caves and/or they were too big (in a different way than 1.18), and since 1.18 I've seen those same complaints resurfacing).
There are also quite a few other features which directly originated as suggestions or mods, including horses, pistons, and ender chests, or at the least may have been derived from a mod, such as many video settings, which resemble many of the ones Optifine originally added (you often hear that they tried to buy out Optifine but the EULA states that Mojang can use anything you make available online, otherwise, nothing stops them from replicating some feature, same for the numerous mods that do the same).