Same for me. Tried on two separate computers, multiple times. Mojang cut support for 1.21.1-1.21.4 within the last couple weeks, so I updated my world + datapack to 1.21.5, only to find out you can’t even upload it anymore. Don’t know how you let this happen….
Can confirm this issue is still happening. Realms recently(ish) cut support for 1.21.1-1.21.4 worlds, so I spent some time updating my datapack to the newest version, only to realize that I now can’t even upload it to Realms. It’s been 5 days, and it still won’t work. Why am I paying for MC Realms if I can’t even upload a world?