The Taft-Hartley Act banned closed shops in 1947:

https://en.wikipedia.org/wiki/Closed_shop#United_States

It's literally the first line of that section.

So why are so many people in this thread complaining about "having" to join a union?

On paper, that means you don't have to join the union in order to get a job - just pay the union fees and work under the union contract (which is to say, almost all the downsides of joining but without any actual say in how the union is run). In practice, I think people who do this tend to get blacklisted by the union and have trouble getting jobs in future. I know the Hollywood unions in particular are very aggressive and public about threatening anyone who takes that option with blacklisting, and there are zero consequences for them for doing this.

As a counterpoint, in my unionized school district, both options were quite popular and you didn't need to pay the full dues, only like 70%.

The law bans employers from requiring people to be union members before they are hired. In states without right to work laws it is still legal to require employees to join the union after being hired.

No. It's legal to require non members to pay an agency fee for the services the union is required to provide them.[1]

[1] https://fee.org/articles/the-myth-of-compulsory-union-member...

So it's still a closed shop in practice? Was the only point of passing this law to confuse the issue?