YAML tags precede their value-terms. For a string value, the !tag syntax occurs before the opening quotation mark/heredoc marker of the string. It’s like a Java annotation.

This isn’t a YAML tag; it’s just YAML embedding some opaque text from some random command-language that uses exclamation marks to mean something.

In fact, actually considering the meaning of the action, the ! used here probably is just the regular ol’ boolean NOT operator in most languages: the action has specified that it should trigger only for commits that aren’t tagged as WIPs. (Which is an eminently-sensible condition to provide.)