I only use Firefox for browser compatibility testing, but I just upgraded from v51 to v55. It actually required a few updates in a row to get to v55.
Anyway, GreaseMonkey is still working fine.
From what I’m reading in that article you posted, it sounds like they’ve only discontinued NPAPI plugins, which is the same thing Chrome did years ago. In fact, that change was done over a year ago in Firefox.
That being said, they ARE going to discontinue ‘legacy’ plugins in November 2017. New plugins will have to use their WebExtensions interface. I read that the GreaseMonkey developers aren’t very happy with the limitations of WebExtensions. They’ve been looking at TamperMonkey as a reference, since TamperMonkey is already a WebExtension.
So… I installed TamperMonkey. Seems to work fine with the half dozen scripts that I have installed… Ultimate Timeline, Dashboard Progress Plus, SRS Grid, Clean Dashboard, Self-study Quiz…