I can't open old 1.16-21w05b worlds

When I try to open an old world (pre-height limit change, 1.16-21w05b) MC says it can't open it because some data packs are bugged. I delete the data packs, try again, and it says the same thing, and offers me to start in "safe mode", and then when I click on it nothing happens; it only allows me to go back to the title screen.