Several comments already mention that the proposal is guided by English Wikipedia and cloned modules and template systems.
There are other wikis who have similar templates, more than a decade for establishing their own classification and issues and maintenance procedures, but other appearance and not a matching distinction.
The proposal needs to be an abstraction from enWP which might be a test case, but not the role model all other wikis have to follow now.
On the level of CSS selectors (classes) a neutral formulation is reasonable, which can be assigned by local template implementations and enWP might map into their system:
mw-page-issue-urgentmw-page-issue-severemw-page-issue-importantmw-page-issue-errormw-page-issue-warningmw-page-issue-normalmw-page-issue-neglectable
or whatever.
A simple three-value distinction called high-medium-low seems to be too simple and blocks later extension by additional levels which might become necessary. E.g. some registered authors who claim to maintain articles in preferences should see error and even warning messages, which are hidden from normal and anonymous users.
MediaWiki software is supposed to identify such <div> or other block element and show the appropriate level in current presentation.