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
OC text by @Fmstrat@lemmy.world
Technically Office Open XML is not proprietary, since they published and standardized it under ISO/IEC 29500. Practically no software suite besides MS Office is able to render it 100% compatible to each other. But to be fair, neither did OpenDocument; a document in LibreOffice odt looks slightly different in OnlyOffice, WPS, SoftMaker, etc.
Huh you’re right, seems they even publish specs of their own extensions to the format.
They still restrict what you can do with it and how you can share it though, so technically still proprietary.