They are definitely an issue for accessibility. I make sure to put "Hey! Don't put anything in this field!" as a placeholder.

That sounds like something bots could easily adapt to if the practice become widespread.

It's already widespread. But, it requires customization to overcome for many spammers. Depending on the popularity of your site, your threat model may require you to do more. But it is still a useful tool.

Though bots may use it, it seems the perfect use case for aria-hidden.