I've played in games that don't allow multiclassing, but never games that don't allow feats. Go figure.
How many published adventures have NPCs which use feats? I think the answer is zero. It's assumed to not be part of the game.
A couple of reasons. First, as I already mentioned, it's really a sop to newbies, not something intended to restrict the choices of experienced players.
Second, feats/MCing have always been, and remain, part of the menu of possibilities of levelling-up, just like every single other aspect of the game!
To quote a cool guy from a few months back.....
" Impressive… Every word in that sentence was wrong."
Some of it likely comes from organized play.
The difference being though, classes ARE an assumed part of the game. You cannot actually play D&D without classes, at least, not 5e D&D as written. For example, magic items are based on class, at least in part. The entire magic system is based on classes. Yank out classes and you don't have much of a game left at all.
I am not sure why you cut and ignored all the language quoted from the PHB stating in no uncertain terms that feats are purely optional and purely up to the DM. But, feats are not optional just to help newbies...as I explained, a lot of very experienced DMs also don't allow them, and several have explained why. Feats have not "always" been part of the game. In 5 editions plus B/E+ versions, they were only expected in 3e and 4e. In 5e, they are purely optional. Stated as such.
As for the debated about the logic of feats reducing options under the illusion of creating them, I opened a separate thread for that topic and linked to it earlier in this thread. Some agree, some disagree, but nobody's been insulting about it and called it absurd logic.