You can point the decryptor directly at the versions dir, while telling it what the folder ID is. The files are however decrypted to their original name, discarding the versioning, so you’ll probably end up with only the latest. You can inspect the verbose output to figure out which encrypted file corresponds to which plaintext file and point the decryptor at a specific directory or specific file only, if that’s what you need.
syncthing decrypt --verbose --to ~/tmp/decrypted --password=p4ssw0rd --folder-id=abcd-1234 ~/some/folder/.stversions
syncthing decrypt ... ~/some/folder/.stversions/6.syncthing-enc/CA/FVS2TU26TV5V82R3OSBKT0JFT0OFKPNBM9406OS\~20210914-132535
to target a specific file. (You’ll get an error as well in the last case because it really expects the argument to be a directory, but it’ll decrypt the file first.)