I’ve just seen that Meta changed the Notification and Hamburger panel, and now they slide from the edges (which is pretty cool)
How can it be enabled on any forum? 
This is all @featheredtoast’s work 
We’re experimenting with the slide-in menus on mobile here on Meta and that will be merged in core at some point in the future so there’s not much you need to do but wait 
you can follow the progress here