This is going to happen a lot, I think, while things shake out. If Beehaw don't trust Lemmy.world to ban toxic users, then defederating makes some sense. I think it's probably self-harming, though, because it'll lead to communities like Technology being replicated on other instances, and those are the big Beehaw draw.
You can always have multiple accounts on multiple instances. A pr0n alt was common on Reddit, and is the only way to get at adult material here. Maybe in the fediverse it'll become common to have a safety alt, and a spicy alt too.
By looking at the access logs. Googlebot sends a user agent string so you can identify it.