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.
You should read this. https://erinkissane.com/untangling-threads
By federating your instance to Threads, you are providing more content to Meta for them to place ads next to, supporting a network that allows abuse orgs like Libs of TikTok with limited moderation.
Consider this scenario: LoT sees a post you made on Lemmy. They select you as a random target of hate, as they did hundreds of people at the beginning of this year.
Their followers start going after you, but you don't know it because you block them. It turns to real life harassment.
This is a real issue, one of many.