Ubuntu 20.10 is EOL, which applies to Ubuntu-MATE 20.10 too.
When releases go EOL, many PPAs get erased; they aren't moved, but contents dropped. This can occur as a release goes EOL, or sometime afterwards (varies on how closely the EOL was noticed & on who does the removal).
To avoid the issue, you should upgrade from a release before it reaches EOL, as the only question after a release has reached EOL is when it'll disappear. There is no new repository for groovy as it's EOL.