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#%EF%B8%8F-legal-note