These questions are not off topic; the update has indeed made no time limit ranked games playable only with friends.
No time limit unranked games (= training mode) are still available with everyone.
OK ... and why is it not automatically ?
I think, if it's the only possibility to play like this, why when I create a game with "no time limit" the restriction "only with friends" is not automatically actived ?
Answer is (as often) time scarcity. Priority was to stop the abuse without spending excessive resources.
Very often we tend to reason like time and resources are unconstrained, but in practice they are.
You can put in a suggestion to quantify interest from the community.
& why is not possible with the private groups ?
That would probably not be too open to abuse, but the rule would be less clear cut (and thus prone to confusion). Also I still haven't seen a clear explanation about why this setting would be preferable to using either training mode or the slowest time setting for turn based. Furthermore, if they are friends in those private groups, they just won't kick you out of the game even if you go over time, so where is the real issue?
Anyway, you can also put in a suggestion about that to gauge interest from the community.