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All Proton Drive apps are now open source
(proton.me)
This is a most excellent place for technology news and articles.
I'd recommend checking out OnlyOffice
LibreOffice can also be self-hosted as a web service.
Libreoffice does not have collab features which Open Office does
LibreOffice Online does. It's not built around it, but Collabora's work has added some level of collaboration to it.
That said, if collab features are your top priority, then OnlyOffice may be the better option, since it was built web-first. I don't know what OpenOffice offers here though.