The license says:
> you must retain the original Product logo when distributing the program
I understand "retain" in the way that you have to display the logo anywhere where the original OnlyOffice displays it. So I think you actually have to "use the branding of OnlyOffice".
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> What Nextcloud did was to remove this attribution
Did they? If including the logo anywhere counts as attribution, I don't think they did. The logo is still present in several places:
https://github.com/Euro-Office/core/blob/main/DesktopEditor/...
https://github.com/Euro-Office/desktop-apps/blob/main/win-li...
https://github.com/Euro-Office/server/blob/main/branding/inf...
They changed it here:
https://github.com/Euro-Office/server/blob/main/branding/inf...