Have you heard of surveillance cameras and facial recognition? If a hostile actor knows in advance that members of a targeted online community will be physically present at a location at a given time, those people will be linked to the community. It doesn’t take a lot from then to link specific persons to accounts.
Besides, libraries are having a hard enough time just existing in America. They don’t need the burden of protecting the identities of dozens of people and fighting off lawyers and enforcers.
I don’t think you fully comprehend just how many footprints people leave behind on the internet. Users would have to practice perfect opsec – and I mean completely, absolutely perfect. One mistake, like using an e-mail address or an alias off-site, will link a person to the account. If that person cracks under legal threats, the entire operation is fucked. It’s happened before.
Thinking you can solve the issue of privacy with a single idea is simply delusional.