Thanks for raising this issue, David.
Tags are restricted because internally they are projects. Project (and thus tag) namespace is global, so if everyone were to create 3 tags, then if we laid them all end-to-end, they would stretch halfway to the moon.
My understanding about the history of tracking tasks is that they were parent tasks, like epics, but with no concept of being "done". Something like "Improve documentation" could have children come and go, but would persist potentially forever. A project (or tag) would be better.
I don't understand "Creating new tracking tasks (tasks that 'automatically' get resolved when all its dependency tasks are resolved) in Phabricator is discouraged", because that almost describes epics or other parent tasks. Those are a good thing, and should not be discouraged. Except for the "automatically get resolved" part, but if epics did automatically resolve, I don't think that would be a bad thing.
Using columns within a categorization project would be an option. And a less-cumbersome option now than it would have been earlier this year, because phabricator now allows you to change a task's column within a workboard by setting a dropdown. (You always used to have to drag-and-drop them).
You could just insert a known string, like [Bad weighting] in the titles of tasks related to that type of issue. Searching would work, as long as you chose strings without accidental collisions. And as long as you were very careful to never misspell one of your keywords.