When you're choosing a domain name or subdomain for your own site or service, it's best to avoid the name of the software it runs on.
If you switch over to different software in the future but carry on using a name that mentions the old software, this may violate the terms of the trademark.
For example, if you use the word "Mastodon" in a domain or subdomain running non-Mastodon software, that might break the trademark terms of Mastodon gGmbH:
@homegrown Fuck the trademark mastodon is this: https://en.m.wikipedia.org/wiki/Mastodon
They only have the trademark in relation to social networks, it's still okay to use the word in other contexts.
@homegrown I’d not looked over that before, but my reading of it is that they don’t want you using “mastodon” in your domain, even if you _are_ running it:
“You may not use the Mastodon word mark, or any similar mark, in your domain name, unless you have written permission from Mastodon gGmbH”.
Yeah, I'm not a lawyer so difficult to comment, but safest is just to avoid the word completely on domains at least.
It's very easy to change the content of a site, but it's much harder to change domains, subdomains, URLs etc so it's best to keep them neutral.