Unfortunately, his son and associates have taken down most of the full length lectures within the last five years, as you probably already know. Other than purchasing the audio directly from his son on his site, I believe they did release many of the transcripts of the full length lectures on his and other sites, so you can still read them.
I don’t recall what year the copyright takedowns began, but about five years ago it became noticeable. In the late 1990s to early to mid 2000s, you could find hobbyists uploading them all over the place. When YouTube got started there was a brief point in time where almost half (or maybe more than half) of all Alan Watts recordings were uploaded. Podcasters like Lorenzo made it even more popular (don’t recall where he got his recording from, but pretty sure he was later served with a takedown notice by the Watts family).