What is every rule surrounding usernames?

no swear words, no pejoratives, no sexually suggestive content.

This also applies to club tags

To add on to what disregardable said, here's a snippet from the Terms of Use, Section 5, Subsection 1, Article XI.

xi. Selecting a Summoner name that is falsely indicative of an association with Riot Games, contains personally identifying information, or that is offensive, defamatory, vulgar, obscene, sexually explicit, racially, ethnically, or otherwise objectionable. You may not use a misspelling or an alternative spelling to circumvent this restriction on Summoner name choices. Riot Games may modify any name which, in the sole and exclusive judgment of Riot Games, violates this provision, without notification to you, and may take further disciplinary measures, including account termination for repeated violations;

So, no Summoner names like "Riot SlayerKing" or anything like that, no using personally identifying information (names, locations, etc.), and no Summoner names that use vulgarity, sexual content, hate speech, other pejorative terms, etc. - and using misspellings to circumvent those restrictions isn't allowed.