I don’t think any normal browser has plans to incorporate any crypto related domains. Brave comes closest with its native .onion support, but even that doesn’t seem to be interest implementing these alternate roots.

Apparently Opera has support for it, but I don't use/like Opera so I can't confirm.

As far as I can tell, practically nobody is running these alternate DNS servers on federated servers, so a Lemmy domain on such hosts would be isolated from the rest of the network.

Yes, currently these domains are useless. A Lemmy instance on such a domain would only work if both servers and browsers support the Handshake protocol natively (without changing your DNS server and stuff). But maybe this will change in the future. Personally I think it would be cool to be able to come up with your own TLDs.

I’m surprised Namecheap and Porkbun even dare sell these, though they do come with the necessary warnings at least.

I was just as surprised as you! I was actually looking for a ".ks" domain last week because I had the same idea, and then I saw that Namecheap had it, only to find out it was a Handshake domain.