Tbh the M1 keyboard seems better than my 2012 MBP (and well, better than that clusterfuck or 1st gen butterfly design, but that ain’t saying much anyway).
Maybe it’s age, but the older one feels mushy
To me 2011 has slightly more key travel than 2015, but 2015 and M1+ seem to have about same keytravel but M1 is a lot "crisper". M1+ keyboard seems to uniformly actuate wherever you press it, whereas the shallower keycaps of 2015 keyboard mean that it feels different when you push it from corner vs center.
This actually reminded me a study 40% of people can't tell the difference between Coca Cola and Pepsi. And then some 30% can taste the difference but can't tell which is which, 10% can tell but don't have preference.
I guess I am in the extreme minority. You could blindfold me and I could tell you by typing which keyboard is which. And this is not to brag but I much rather I don't have these high standard. Life would be a lot happier.