It just joined the musescore project, great open source music notation software. For funding the only commercial thing they offer is a site where you can upload & download scores, with the paying part also paying licensining fees for copyrighted music. Imo all looks very legit. I was already familiar with musescore before this drama, and watched some of tantacrul (head of the musescore project, and now also audacity i guess). He's a very down to earth guy that has quite some insightful videos on the musescore development and figuring out what to keep/remove when going for new versions. But also great videos regarding other topics.

So far i've seen nothing that rings any alarm bells. The open source community can sometimes be a bit too sensitive regarding paid services linked to open source software. But in this case as long as the actual software remains open source, and the paid part actually adds value (a nice place to exchange sheet music, without any copyright issues as that's covered by your payment, so a very legit reason to ask money), why not?