It looks like they may have temporarily pulled it down, possibly due to problems with Arm. There are some issues on their developer forum that the latest SDK (2.4.0) is not building properly in Xcode. Suggest contacting the POC who set up your Avid account and ask them what’s up.
For what it’s worth, AAX SDK version 2.3.2 builds as Universal Binary on Xcode 12.5 which is what I have been doing for some time. I don’t know if it works because there hasn’t been a way to test it until recently (I understand there is now an M1/Arm version of Pro Tools that runs on Monterey - and ONLY Monterey). Will wait for any complaints before I address, recompile and re-issue everything (again).
l thought AVID was supposed to certify your programming skills before handing you their SDK? . That’s what’s been keeping me from even getting near AAX and Pro Tools.
Certification is separate from getting the SDK. That said, they have changed policy over the last few years - it used to cost a not-insignificant amount of money annually to get and keep access to the SDK. They seemed to have decided it was more useful to have more plugin vendors than fewer serious/professional ones.
Also FWIW the SDKs and Dev/Prerelease versions appeared to be back when I checked yesterday.