The FAQ page says:
Do any sites have custom hats?
No. All hats can be earned on any participating Stack Exchange site. There are also a few hats that can only be earned through meta site participation.
The 2cool4skool hat description says:
Try out the High Contrast or Dark Mode features (Stack Overflow only).
I propose the FAQ page be updated to include a caveat to say that the 2cool4skool hat can only be earned on SO sites and international SO sites.
Catija explains in the comments:
we don't make hats just for specific sites, they're for features
I propose that the FAQ page be updated to state that some hats may be awarded for trying features that are not yet supported on all sites (and can only be earned on supporting sites).
What Catija suggests in the comments along the lines of "'can I earn all of the hats on every site?' -> 'no, not all hats can be earned on every site'" also works (but seems a tad vague if that's the whole gist of it and there's no more elaboration).
