It is the new iteration of the “watched tags” feature.
See this meta: New post summary designs on site home pages and greatest hits now; everywhere else eventually
Previously, questions with a watched tag were given a slightly darker background shade (see my answer here for reference), and now instead of that nice, subtle highlighting of watched questions, ...
What is the rush?
Intricate tag navigation, remapping and synonyms is something that a tiny group of users does — suggestions end up there where the ten people who care about the feature eventually learn about it when there is some action that requires attention. When no action requires attention, then that is great - if a synonym sits there for four years, ...
This was proposed some time ago on main meta: Some kind of notification system for when tag synonyms are suggested
If we wanted to get it reviewed, I suggest we should get evidence together that it's needed network wide (i.e. not just here), post evidence as an answer on that post, and the flag it for review.