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.
Honestly, even if I agree in principle, the fediverse has exhausted any sympathy from me given that there's consistent toxicity towards people who don't "get it", people who aren't open-source tech nerds or fediverse evangelists. There's this constant smugness that the fediverse and its community is better than everything else and it has no problems or the problems that do exist is how the fediverse should work. No, not everyone is like that, but the ones that are make the fediverse experience that much more painful and I'd rather just use Bluesky, even though it's janky and much more limited in features.
Not even talking about Threads, either. I think this issue is quite prevalent on Lemmy but I remember Technology Connections having this issue on the Mastodon side to the point where he got angry too.