The panic I had a while ago when I tried to use it and realized it wasn’t working anymore… Anyway, it’s still in the Glyphs Scripts menu. Only the name seems to have changed:
I think, it is Adopt Background now, instead of Mask to Master.
If, like me, you use it with a shortcut that you add to your system as described in the topic below, changing the name there should be enough to get it working again.
BTW, I would like to thank @TimAhrens for this. It must be one of the things I use the most when using Glyphs, because when I lost the function, I felt as if the rug had been pulled out from under my feet! — everything is fine now 
Sorry about the upset, I was aware of the shortcut problem but the name really felt too inappropriate (it seemed right when it emulated the old FontLab “Mask to Master” command). Glad you like it – it’s also one of the most important tools in my daily work (particularly in combination with “Insert Glyph to Background”).