That's not a valid top level domain (TLD). Only specific letter combinations are valid. See: https://en.wikipedia.org/wiki/List_of_Internet_top-level_domains (technically you can apply to create a new TLD, but there's a lot of rules and it costs hundreds of thousands of dollars).
The closest existing TLD I can think of is Norway's .no (lemmy.no).
SFW only for any community on dbzer0, but you can post and view nsfw when visiting other instances. We're a pirate instance so I assume the admin don't want to attract more attention by including potentially illegal porn. Idk why lemmy.fmhy.ml allows it tho.