OnlyOffice appears to have removed their Android app from their repos: https://github.com/ONLYOFFICE/documents-app-android
It can still be found on an archive: https://archive.softwareheritage.org/browse/origin/directory/?origin_url=https%3A%2F%2Fgithub.com%2FONLYOFFICE%2Fdocuments-app-android
This means if you are de-Googled like me, they are no longer an option.
Perhaps coincidentally, they have also started making legal claims against the EuroOffice fork: https://github.com/ONLYOFFICE#️-legal-note
Sort off. The main body of OO is, but there are binary blobs for some MS compatibility, etc. This is why EuroOffice has started as a fork.