Ive been noticing over the last several days that every time a new experimental build is posted that one or more mods is listed as being removed. Are all these mods being permanently removed or do they no longer work with new versions of the game or something?
Some get obsoleted, some are no longer maintained, content of some mods got mainlined
Use this modpack, this one works perfectly fine with 10644 apart from minor negligible errors that can be easily Shift+i ed, oh and use @TheGoatgod 's mod compilation, it’s our Gold Standard, he works really diligently on it 
https://github.com/TheGoatGod/Goats-Mod-Compilation
The modpack that works with 10644
https://mega.nz/file/KMF1BCjY#tDSS89PLT4xYbvJw53MRBtmdXRexisvjlDceN6k5Tuw
Like Kenan said, they were obsoleted largely because most of them were orphaned for years and/or poorly implemented, especially compared to modern code standards in the vanilla code, or have already had anything usable assimilated into the vanilla experience. Some stuff like toggles just don’t need to be provided as a default option when the features are no longer buggy or half-implemented. That doesn’t mean users can’t find them as user mods like how goat’s compilation is being hosted.
Is there a way to add mods into a game world that is already created? Or do I have to start a new world/game to add mods in?
You can use the forum search or Google for stuff like this. But anyway, choose one guide:
I love when people just automatically assume that you didnt try finding the info before asking? Wouldnt it stand to reason that I would first look for myself rather than asking on a forum and waiting god knows how long for a possible response?
That being said, Im sorry that my search-foo apparently wasnt good on this occasion, but thank you for helping me anyways with the info you provided. Try to keep in mind though, this game, for many people, myself included, while awesome, is not very intuitive and overloaded with info. Overwhelming even. It can be easy to miss things in the crush of information even when searching forums and Google.
I love when people just automatically assume that I assume things when all I did was to state that, and I cite myself, “You can use the forum search […] for stuff like this.” which does not contain any other info or “assumptions” than what it says on the tin.
Try to keep in mind; my posts usually also target others searching for info or randomly stumbling across this post and may didn’t consider using those methods to find information on the game.
Anyway, you’re welcome 
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Did “no reanimation for zombies” got obsoleted too?
Yes, the Prevent Zombie Revivication mod (ID: no_reviving_zombies, mod folder No_Reviving - in case you want to… deobsolate it) was obsolated (unclear why, maybe because no one maintained it).
The mod pack linked from Mega up thread, was that different than GoatGod’s current experimental pack?
I’d like to try helicopter fun, but as experimentals are hit or miss for mods, and there’s stuff that’s being obsoleted in terms of items/etc, I’ve been playing Stable 0.E.