I feel like this setting would be a lot more useful if it was per community. Plenty of sports communities use bots to set up gameday discussion threads that I don't want to miss, but I DO want to miss the ocean of garbage reddit reposts drowning out actual human interaction.
Which isn't a critique of your very relevant suggestion, but rather a lament that people think that communities somehow get better when robots are posting junk no one reads.
What is your definition of bot? Even if you toggle the "I'm a bot option" off you can still access the API using a programming interface. Thus you are able to run a bot even if its account has not been defined as a bot.
It is probably possible to create a client (like Jerboa or Lemmy-UI) that is able to hide messages from accounts that are defined as a bot. But if a account hasn't been defined as a bot, its messages will still show up, even if they are written by an automated script.
I get why you might want to, however there are a decent number of bots right now working on posting up content to Lemmy from either reddit or a stockpile somewhere. I'm working on a bot right now for my private instance that would post a few stores up pictures a day of my dog until it exhausts it's supply just to start stimulating my feed more. That's just an example from me, but the way I see it bot content posters aren't the worst for the health of Lemmy right now as it continues to grow.