Add the tag to questions which are about the ruleset
Adding and/or correcting the tags of old questions is useful. The only caveat is to not flood the homepage with such edits. There's no hard rules for what constitutes flooding, but spaced out clusters of half-a-dozen should be safe.
But a question mentioning "Path of War" doesn't always make it about Path of War. Specifically, a question about a character option from that book might mention it, but doesn't need the tag. There is in fact guidance for tag usage in the excerpt:
Use this tag on questions asking specifically about the overall supplement or issues specific to the ruleset, not simply on any question about specific character options from Path of War.
At a quick glance (admittedly from someone not familiar with the system nor book), it looks like several of these are about the ruleset and do deserve the tag, but not all.