Pixhawk 1 is unfortunately past its End of Life so can’t be made anymore, which means it’s generally only sold as part of kits that require it, out of previously bought stock. Understandably that’s frustrating for everyone, and the Blue Robotics team is working to make an alternative board available with proper future support and an improved feature set.
In the interim, if you’re unable to obtain a Pixhawk 1 we expect that the Pixhawk 4 is likely the next best thing. I asked internally and was told we haven’t officially tested with the Pixhawk 4 (hence it not being in our confirmed working list), but because it’s intended to be basically a drop-in replacement it should likely ‘just work’ with ArduSub. The main components of “we don’t support it” are basically
- our current documentation is geared to Pixhawk 1, and
- the Pixhawk 4 has some additional features that aren’t implemented in ArduSub, so while ArduSub should work on it, it might not be able to make use of all the hardware that’s actually there (e.g. a couple of extra auxiliary IO ports that weren’t present in Pixhawk 1)
As a note, @hydropumpon was recently asking about adding a leak sensor to Pixhawk 4, so perhaps they can confirm if it works for the rest of ArduSub 
Yes, unfortunately the ArduPilot pre-built binaries page is only available through http, not https, so it’s unencrypted. That’s much more important for sites that you actually have accounts with and whatnot, but it’s still not ideal.
https became the de facto standard within the last several years, because it’s better for basically everything, but some sites are yet to update to it. The whole internet used to use plain http, so it’s not like every http site causes a guaranteed virus download or anything, but browsers now warn about it because it is technically less secure (higher risk that something bad could happen), and the warning is something of an encouragement to website maintainers to update their pages.