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When/by whom was it rejected? Would be helpful if you could point me to the source of this claim.

I don't know when it was rejected or who did it, and given that dev replies here aren't always dev-flagged (which means they don't show up if you filter out non-dev posts), that it could have been rejected elsewhere (some devs have occasionally engaged in modding-related discussions elsewhere, and it is conceivable that a rejection happened directly on the wiki page since the volume of suggestions here is large enough that replying to everything is unlikely to be reasonable), and that the search functionality is broken (as an example, searching for Meneth's posts in this thread shows zero results after 2015, but looking at the dev replies you find several from 2017) I'll not go hunting for the original rejection.

However, it is listed on the wiki as a rejected suggestion, and the wiki page in question is somewhat monitored (at the very least there's a "Don't remove anything without providing a source or we'll revert it" note), so I doubt it is there due to vandalism. Also, this post is at least somewhat relevant; while that post isn't an outright refusal it is quite unlikely that anything that would take a lot of time (which I think is a fair assessment) is going to be done in CK2 at this point, considering that CK3 is supposed to be released this year and that an announced sequel generally means that the previous game's development dies down.

If true, this is very unfortunate since afaik even if theoretically one could replace a hard-coded diploaction with a custom targetted decision, it would be very difficult if not impossible to replicate the UI windows/elements the former make use of (for example, can you mod in the favour button to windows and actually make it usable in conjunction with a custom targetted decision?).

As far as I know you can't mod in the "Force this with a Favour" button on a custom targeted decision, though making it possible to force through a targeted decision with a Favour would probably be doable either by having two versions of the decision (one spending a Favour, one asking nicely) or by firing a "Hey, do you want to force this with a Favour?" event when you take the decision. Of course, without more knowledge about the AI's reasoning you'd have to guess as to when it should call in a Favour to make X happen...
 
Patrilineal and Matrilineal disinheritance.

I am adding a mod which allows commoner dynasties with surnames, so it's possible that a commoner man patrilineally marries a noble girl who might even own something. As the children themselves will be commoners due to Salic law unless the ruler ennobles them, they shouldn't be able to inherit the noble fief of their mother, but of course they will get whatever their father owns. On the other hand, it would allow for morganatic marriage: if a poor Prince marries a rich merchant's daughter, he and his children might be excluded from the succession to the throne and any territories, but they will of course get the money of the mother.