After I posted a twitter critical of Joe Biden and his administration, my relation to Joe and Jill were altered to eliminate my relationship to both of them. Is it possible to detect who made those changes?
Yes, it is. Change log is down the right side of the page. BUT since you are (apparently) still living, your profile is not public. That means no one, unless they have access to your account or helper profile, can change your connection.
I gather you made your own private PIDs for Joe and Jill Biden in your private tree space, and they are related to you through persons who are deceased. The deceased persons are in the public tree space.
Somewhere on the path(s) through the tree from you to them, one or more relationships was deleted. Do you have a copy of that path, or know the person ID of a most recent common ancestor? I ask because to find the deleted relationship you may have to traverse the tree between yourself and them. If the tree has been well worked out, however, there will tend to be more than one path between two related persons.
By way of example, I know two men with the same name who are 7th cousins once, and also 8th cousins a handful of time, and 9th cousins many times. They both descend from multiple families resident in a certain village 7+ generations ago.
Once you find the change, you will know the display name of the contributor who made the change. That information may or may not be useful to you. You will be able to message the contributor.