I am creating this thread in an effort to compile all known support conversations that have some sort of notable time-lock on them. By this, I mean particularly:
1) Support conversations that can be missed completely because they are only available pre-skip (i.e. before the end of Chapter 12). If you do not get one of these supports when it is available, you cannot continue the support chain past that point
2) Notable support conversations (like Byleth B-supports for recruiting purposes) that are locked until after the time-skip
There are three general rules that apply across the board:
1) With the notable exceptions of Alois and Gilbert, every character's C-support with Byleth is only available pre-skip
2) In addition, all A-supports are locked until after the post-skip, with the exception of Byleth x Rhea, which MUST be obtained pre-skip
3) To obtain an S-support between Byleth and another character, you must obtain the wedding ring from Jeralt's room when prompted by the game during Chapter 10. If you do so, on the very final day of your route, before the final level, you will have the option of choosing which character to S-support from a list of all compatible characters you have an existing A-support with.
Here are all the other support conversations that have some sort of time-lock that I have noticed (list is hidden due to length):