We are checking that styleTag.current is defined few lines above so it's not needed, right?

i thought so as well, but linting was throwing an error

I think the error was because it's a ref, so styleTag.current could theoretically be changed/nulled by other code before the cleanup function runs. In this case, you're not using the styleTag ref outside of the effect anyways, so you could delete the ref and make it a local const instead.

  useInsertionEffect(() => {
    const styleTag = createStyleTag(targetDocument, styleTagId);
    if (styleTag) {
      insertSheet(styleTag, rule);
      return () => {
        styleTag.remove();
      };
    }
  }, [styleTagId, targetDocument, rule]);