What I mean by alternate earths is taking current day earth geology, and then applying different realities to it.
- a geographically identical earth but with a machine based civilization
- post apocalyptic earth
- zombie apocalypse earth
I would say yes, as long as it pertains to the world building (and thus is a fit for the site's scope in the first place).
The site is defined in Area51 as
[a] Q&A site for writers/artists using science, geography and culture to construct imaginary worlds and settings".*
I see nothing there that would suggest that only non-Earth worlds and settings should be allowed.
There are several questions on the Area51 proposal which are fairly highly voted and more or less explicitly deal with humans or our Earth. For example:
Also, lots of fiction works take place in worlds that greatly resemble our world and society, with anywhere from trivial to fairly large differences. We wouldn't want to bar those questions here.