@vruz Have you read the posts at all? It's not about Threads.net or closed systems.
It's about in this case really bad UI. If you search for an existing account on a for-you-blocked instance, it should not say:
"This account doesn't exist" (which is just wrong)
but:
"Could not find this account. It belongs to an instance which your instance has blocked. The reason given is: XYZ" (True and helpful)
That is a Masto/Fediverse problem.