Imslp is a free library of public domain music. I too find their fundraising methods distasteful, but full resolution scans take up much more server space than plain text, so I understand it. Imslp remains the best source for public domain scores.
MuseScore is an open source project. Due to its license the software itself must be open source perpetually, the new owners have used it to push their proprietary network to make money by hijacking a free software project.
I have musescore 3 on my computer because version 4 is such a step backwards.