Yes @0x7 had noted some had stopped working and had updated them where he could - but likely more have also. It’s not so much they are defunct, just the link has changed so need to find the new one.
Another example is the Brave Browser repository for F-Droid:
This post below has the old outdated Brave link in it but also has some great info too for adding F-Droid external repositories (including a list of many common ones):
EDIT: removed references to out of date Brave repository link as URL is the same as one in original post (there was just some transient issue at the time I used it).