Here is the patch to extend DR 526 to forward_list and list remove_if 
and unique.

As the adopted pattern is simpler I also applied it to the remove methods.

     PR libstdc++/91620
     * include/bits/forward_list.tcc (forward_list<>::remove): Collect nodes
     to destroy in an intermediate forward_list.
     (forward_list<>::remove_if, forward_list<>::unique): Likewise.
     * include/bits/list.tcc (list<>::remove, list<>::unique): Likewise.
     (list<>::remove_if): Likewise.
     * include/debug/forward_list (forward_list<>::_M_erase_after): Remove.
     (forward_list<>::erase_after): Adapt.
     (forward_list<>::remove, forward_list<>::remove_if): Collect nodes to
     destroy in an intermediate forward_list.
     (forward_list<>::unique): Likewise.
     * include/debug/list (list<>::remove, list<>::unique): Likewise.
     (list<>::remove_if): Likewise.

Tested under Linux x86_64 normal and debug modes.

Ok to commit ?

François