I’m surprised ONLYOFFICE isn’t more popular in more distributions. It’s AGPL, open source, and almost infinitely more intuitive than LibreOffice to anyone coming from Microsoft Office.

Are the desktop apps open source too? I have a faint memory of only their document server being open source or something like that.

Sure are.

https://github.com/ONLYOFFICE/DesktopEditors

The headless server is Apache, the client and visual elements AGPL, which might be the confusion.