OP, is it possible to enable this feature in ROM? :)
I'm not aware if OP is comfortable with this feature, but it is just a boolean switch away. :fingers-crossed:
There's no google implemented Clear All button if you're referring to the overview menu (multitasking). Nexus 5X user here (previously owned a Nexus 4 with this ROM).
Not multitasking but actual Clear all recents button. Read here ht...
While I was updating (as per OP instructions), I flashed 10-19 over 10-15 build, then rebooted (Frank bootanimation, GNL replaced by stock launcher, no Google Play Services, and other Gapps issues), then rebooted into recovery and flashed Txuki Ga...