Our tags are normally using the plural, especially when they are about a grammatical category.
Yes, I agree. When our tags have the form of a countable noun phrase, they should be plural.
Tag names should have a generic interpretation. That is, when we use the modal-verbs tag, we're saying "This question is about modal verbs", referring to the class of modal auxiliary verbs in general. And although there are several ways to form a noun phrase with a generic interpretation in English, because we don't use articles in tag names, the only relevant option is the plural generic, as in the following example:
Modal auxiliary verbs are a special type of auxiliary verb used to express meanings such as ability, likelihood, permission, and obligation. These include will, would, can, could, shall, should, may, might, must, ought, need, and dare.
In this example, I've used the plural form to refer to the entire class of modal auxiliaries. We should do the same thing with our tag names. When I see a tag with a singular name like modal-verb, it feels off to me, like the wrong form of the word has been used.