If we’re talking about online editing, Collabora has web editors based on LibreOffice but with a modern UI: www.collaboraonline.com
They are really great and can be self hosted (e.g. with Nextcloud).
For offline editing, as already mentioned LibreOffice has an optional ribbon UI and OnlyOffice looks pretty modern as well.
kossa@feddit.org 2 days ago
Open Office =/= Libre Office
The latter is a long standing fork of the former and considered the “FLOSS standard office suite”. It has a “modern” ribbon UI, but you have to set it like that, it usually comes with the “old layout” by default.
OnlyOffice is another office suite which brings a ribbon UI by default.
While not open source Softmaker Office mimics Microsoft Office the closest (and claims the best available MS file format compatibility, but I cannot say anything about that).