musescore-downloader (via Github):
"Download sheet music [...] from musescore.com for free, no login or Musescore Pro required"
Note: This is not about pirating, but (among other things) about providing access to public domain and Creative Commons music hidden behind a paywall and subscription enforcement. See repository for more info.
https://github.com/Xmader/musescore-downloader
There are Chrome/Firefox extensions and a user.js approach available.