Pre-ban mags can no longer be bought/sold. The only way you can get them is by inheriting them or owning them on 8/1
There's an obvious huge enforceability issue since they are an unserialized and untraceable item, but that's what the law says. My assumption is that, in practice, people who had an LTC before 8/1 will not be subject to enforcement on this, while people who got an LTC after 8/1 will be, as there's no way they could have legally owned them on 8/1. This is just an assumption, but I think it makes sense. YMMV, IANAL.
Either way the mags wont have much value, as no one is going to be openly selling them.
Polymer mags like glock mags are 30+ years old now and the plastic does break down over time. Aluminum and steel mags have a much longer life span.