Both NEA and AFT are on a steady decline here, as a result.
There's no real reason to join for even sympathetic teachers. It's really a symbolic act, at this point. I belonged to AFT at one point and the meetings were rather pathetic, with our district union leaders reduced to to pleading with district administrators for small changes or a couple seats at a table with administrators, while holding no cards.
Even in states that haven't taken these measures, you will see fewer and fewer new teachers willing to give a share of their paychecks to the unions. Many/most of the Boomer/early Gen X teachers who were hard sellers of the unions are retired or are retiring.
Link: https://en.wikipedia.org/wiki/2011_Wisconsin_Act_10?scrlybrkr=bdabda9d