Based on data from https://fedipact.veganism.social/ it seems that the majority of instances blocks threads.net. I'm sure there's Lemmy instances with either approach that have slipped through the cracks as the list is a work in progress.
The percentage of users doesn't correlate to instances as the biggest instance on the Lemmyverse has roughly 3x the users of the second largest, a NSFW instance, 4x the users of the biggest "niché" instance and 5x the users of what I see as the second largest general purpose instance.
Now if only lemmy.world would follow.
I hope not. I know this is controversial but there’s literally no benefit from defederating. People can simply block threads at a personal level if they want.
Even if you do, everything on Lemmy is public so you gain no privacy advantage by blocking it. The only thing you might prevent is a reduction in content quality, but again, if that bothers you just block the instance yourself.
This is a classic kneejerk reaction where people are trying to prevent a hypothetical harm before it has even proven to be a problem.
I can tell you one benefit: Money. Most of my server's costs come from storing federated content. Federating with threads would likely be expensive.