It is possible to add tags to questions created when making a Quiz. The feature is due to be implemented at some point in the future but does not currently work. See the forum discussion https://moodle.org/mod/forum/discuss.php?d=194119
Can you tag courses?
Managers and teachers can tag courses in Course administration >Edit settings, and all users may search for them in Navigation >Site pages > Tags.
Other users may be given the Course tagger role to tag courses from the page Course administration > Course tags.
When someone adds an unsuitable or inappropriate tag, other users can click on the word (1) to get to the screen where they can flag it as inappropriate (2) The administrator can modify or delete inappropriate tags from Site administration>Appearance>Manage tags (3)