As noted at T354153#9429017, when the User class was namespaced, it is no longer hyperlinked by Doxygen.
Before: https://doc.wikimedia.org/mediawiki-core/REL1_40/php/interfaceIContextSource.html#abb34a4e515bd16d9b32ad12bd893a073
After: https://doc.wikimedia.org/mediawiki-core/REL1_41/php/interfaceIContextSource.html#abb34a4e515bd16d9b32ad12bd893a073
The same seems to be the case for various other namespaced target classes.
This decreases usefulness of the docs, and makes browsing classes harder