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.
Mastodon, which has far more users than lemmy, implements "authorized fetch", which can actually affect whether Meta can just crawl it.
I know we're on lemmy, but it's not the case that anything on ActivityPub is necessarily fully public, or that instance-level blocking is identical to user blocking.
You’ve only made the case to implement authorized fetch on lemmy. Mastodon did it, Pixelfed did it. No reason why lemmy cannot do it.
edit:
To be clear, I don’t give a rat’s ass what lemmy instance admins do in this matter. Threads interaction is more compatible with mastodon and pixelfed anyway. I just think the chicken little act is ridiculous when bad actors of concern are definitely already here. You’re not stopping anything from coming in. This place isn’t untainted like freshly fallen snow. It’s a pool that gets peed in on a regular basis.
Yes, lemmy should do that.