They do eventually go back and ease/remove these restrictions. The timeframe varies, but it does happen. Bosses that are ‘optional’ tend to take even longer to be nerfed than required ones (or ones that drop the high-value stuff like mounts), but it does still happen. Eonar in Antorus was a pain point for a long time but it’s been addressed fairly recently for solo players.
If we’re being honest though, these nerfs probably rely a lot on if and when anyone on the dev team is running legacy content on their day off and runs into a boss they can’t solo lol