No there's no rule against it and it's not unethical at all.
If an official clan needs members then it's got as much right to advertise that as any other clan.
It's how I first got recruited for raid
She/They
Yeah, I only don't like erthtkv2 because of the mod's name. Make it "tkv2," and the mod will instantly become more popular. This is a valid reason as the name of the mod is still an important feature that no one seems to have yet discussed.