I used to be a big fan of the Samsung Gear VR. I used it with my Galaxy S10+ and even for watching things like movies as well as playing some VR games. However, it has been common knowledge for around 2 years that the S10 was the last phone to support the Gear VR. I believe Samsung and Oculus decided to part company, and therefore any further phones after S10 were unsupported.
Of course the S21 Ultra would never fit in the VR headset form factor, and if having researched it properly then it should have been realised. The S20, S21 ranges also do not have any support for the Oculus store or software, so even if you were able to connect it physically into the headset, then it will not work as the software cannot install.
I am also disappointed with the fall out between Samsung and Oculus and found this to be a nice toy. However all good things come to an end... and I have invested in an Oculus Rift S for my PC to get my VR fix.
p.s. the S21 ultra is awesome, but I do miss my quiet night movie showing through my VR with a virtual cinema room and Bluetooth headphones...