Maybe sprinkling 1337 into our online texts would make them less useful to AI training

No, it wouldn’t.

I think search engines didn’t work with it;

No, they worked fine. It doesn’t take much for a computer program (especially search engines and chatbots) to recognize two words spelled differently have similar meanings.

Go misspell anything into a search engine, whichever you use, and notice how it suggests a correct spelling.

So if a few people try this, it’s not enough to have an effect. If enough people to have an effect do it, then the program quickly learns leetspeak.

But that doesn’t stop people from constantly having this idea

Maybe it will, maybe it won’t, but either way: Please, no. One of the most refreshing things was noticing the internet had largely matured from that. I’m all for throwing a wrench in the gears of the AI brains, but there’s a limit.