If each instance decides on their own usage, then it no longer becomes a useful filter. We already see this with the NSFW tag (god forbid I ask for it to be put on a post showing the business end of a fleshlight hanging out the rear end of a stuffed animal dog, that isn't nsfw apparently because it's not real nudity or something).

From one coder to another: This is classic coder overconfidence. The complexity isn't in the hypothetical code but in the people and how the feature may or may not be used.

Explodingheads is going to have a distinctly different idea of "political" than Lemmygrad.

The Fediverse has few enough daily users that you can block political communities and the people who post overwhelmingly political content outside of those communities.

You can also use one of the handful of clients that allow keyword filtering.