1) requires metaknowledge - what number should I use to have a good experience?
Well, its not that hard imho to get there some note saying "
recomended amount for Normal gameplay difficiulty: 40, for Hard: 80" ...
Would it?
No metaknowledge required.

Also, as stated abowe, i think theese things should be adjustable any time you want ...
So if you feel like you have too much food left, and wish to use more ... you adjust it.
If you feel like you underestimated this limitation, and wish to use less ... you adjust it.
This way anyone can get exactly what they want.
For example:
Icelyn stated several times that all Long Rest limitations bother her (or something among those words) ... and that she would rather have none, as it was in first patches.
So she set it to 0 ... and voila! She can rest any time she want.
Maximuuus stated several times that he dont want to hoard food, in order to rest ... so he set it on double (and potentialy even more if he would feel like its not what he wanted) ... and disable food, so the only resource he would have for rest will be camp supplies bags.
And voila! He can rest as little as he want!
I state that im super über badass skiller, and set it on 999 (or w/e will be maximum) ... i quite fast find out that im actually not ... so i just open difficiulty settings, reset it to original value to remind it was 40, and incerase it a little more, jut not so drasticaly, to 100 prehaps.
And if i get to same situation, i shall repeat the process untill i will be satisfyed.
And voila! I can rest as little as i want!

Or i can just leave it alone on default value, and decide for myself if i wish to rest allready or push a little futher ... just as i allways did, bcs i dont really need the game to regulate this for me.

Assuming most people won't even do one full playthrough it is a very nieche option
Thats not a problem, each and every setting option have some default value ...
If you are not sure, just leave it alone.

Or reset it later.
2) Some other RPGs did it, and it wouldnt' change a thing - really
Im aware, but this isnt even supposed to change things.

Its supposed to give players, who wish to use *THIS* kind of limitation for their Long Rest, option to have it.
Sure people can teleport anywhere they want ...
Sure people can just return to previous save spot ...
But then they can aswell just not activate this.

Especialy since they also *can* turn it off any time they want.
I mean ... theese all 3 are just options for people who desire such things.
Presuming someone who activate option that dont allow him to rest anywhere they want ... dont wish to rest anywhere they want ... is just common sense, isnt it? O_o
3) Again, metaknowledge - how do I know how often I should set the rest, without playing.
Same reaction as in point 1.

Also not a good way of limiting rest. Not each encounter is equal. Also what is an encounter? If I ambush and kill a singular sleeping goblin does that count? If I want to rest before big encounter does it mean I need to backtrack and farm encounters to do so?
Not my place to decide, im no developer.

Whatever Larian would see fit tho.

:P
It can be anything from entering combat, to amount of killed enemies by your party.
I wouldnt call it enforcing ...
Not in case where you can turn it off, or change rules any time you want.

This is more like ... automatic way the game would keep track for you on limitations you set on yourself.

//Edit:
Ok ... can moderator move this post please?