Feature summary (what you would like to be able to do and where): \operatorname* is used to define operators like with \operatorname, but with limits typeset below the parameter. Currently, MediaWiki (incorrectly) parses this as \operatorname{*}.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution): Found while trying to fix the spacing at https://en.wikipedia.org/w/index.php?diff=1095145412 (see line 95) and finding that the missing syntax was worked around using a clumsy \underset{V \subset U}\text{colim}, which I had to wrap inside a \mathop to fix the spacing. If this is implemented, it would be written as \operatorname*{colim}_{V \subset U}.

Benefits (why should this be implemented?): This is more or less a standard LaTeX math command; don't see a reason why this wouldn't exist.