DOn't want to ruin your fun, but both of those already exist
I filter specific words/sentences with "Prat" and it allows me to even select where i want to block it. So instead of blocking something globally, one can block specific words on specific chat types (whisper/guild/channel/say/yell/raid/party). And yes, Prat is a whole chat enhancer, so if the person just want this specific feature that'd be a bummer i guess. But even then, the add-on BadBoy and ChatFilter exists, too. I still prefer Prat because the feature is much more advanced either way, be it in comparison with ChatFilter or in comparison with BadBoy (though ig BadBoy is focused on anti-spam, but i don't even experience spam in WoW since i have Prat and i can block specific annoying words on Global chat)
As for the add-on that blocks opposite faction pollution on Dalaran, that one already existed too but i forgot the name right now
Also Prat allows you to do even more than filtering, such as setting sound notifications for specific words/sentences/text as soon as they're detected, which means that even while alt-tabbed, i never miss any raid that i'm looking forward to do