Don't know if this is possible, nor if it has been suggested before, but I was thinking while doing a little tag cleanup about a possible tag relationship that is the opposite of an implication. This relation would disallow any tags that would be nonsensical to have in the same post. For example, the tag "pokemon_(species)" would counter-imply the tag "not_furry", or "anthro_focus" counter-implies "no_furry_focus". I would love to see it as a feature, but feel free to bring up any problems that could arise.

There are plenty of pokemon that aren't furry, gardevoir for instance.

There would be very few of these that could possibly work, because of things like multiple images.

There are very few tags which are mutually exclusive on the same post. This is primarily due to multiple_images type scenarios

If you take a look at not_furry you'll see that it includes humanized characters. So you could make a human/oid version of the pokemon and it won't necessarily be not furry.

Also there's the problem of antimplications working in the background, so they aren't even aware they did it, which is going to be especially bad if they accidentally added an incorrect tag which forces out the other

It's not going to be particularly useful and will take a bit to code in

Should you block adding "not furry" or remove the "furry" tag? It would not be possible to decide which mutually exclusive set of tags is the correct one automatically, an experienced tagger would need to look at the post anyway.

The exception would be aspect ratios, but those can be handled by bots