Hi @l.faye,
That’s correct - they’re past end of life so are generally only sold in pre-packaged kits and the like that require them. We’re working hard on a replacement, which I believe should become available in the next few months.
That’s unexpected - how are you connecting them? As far as I’m aware all Pixhawk versions should be QGC compatible (on any OS) - you’re more likely to run into issues with ArduSub not fully supporting them because it’s been primarily designed around Pixhawk1. As far as I’m aware you should still be able to flash them with ArduSub, but there may be some issues/missing features (e.g. see this post).