Since forever:
SPACE = Jump/Fly Up/Swim Up (Baseline function)
CONTROL = Fly Down/Swim Down. (Baseline function)
I noticed in 12/02/2025, CONTROL Key does not Fly Down/Swim Down anymore, both in Anniversary and MoP. What happened?
Always thought it was page up and down for those, but using right click and hold also works. Think MoP did get recently updated, might want to ask WoW Classic > Mists of Pandaria Classic Discussion