I’ve done Legions 100% solo. So no, players aren’t required.
Helltides definitely don’t require other players, rofl
If you mean any of the Echo bosses; again, you can solo each and every one. I can link a video of The Seneschal solo’ing Duriel if you’d like.
You’re conflating efficiency and mandatory. In an MMO like WoW, you aren’t clearing the latest raids or even dungeons solo. You must group.