Why is there still no support for add-ons for the containerized HA install options?
Especially since there is a shortage of chips/rasp pis, imo it would be incredibly useful to have a fully supported containerized option for HA.
Are there any plans to add support?
What are the challenges?
No.
I laid them out here:
And in my and others responses below within that thread.
I’m not really sure what the chip shortage has to do with HA not supporting add-ons in a non-supervised install? 
As do I. I just created a VM and installed HAOS instead of adding a container.
Well, due to the chip shortage, it is more difficult to find reasonably priced compute to HAOS. I find docker to be a simple solution for almost any setup. In my case, I have a headless ubuntu server that would be ideal to run HA on, but I want access to add-ons.
If you want add-ons then just run a supervised install of HA on your Ubuntu machine.
It still has nothing to do with a chip shortage.
You can run a supervised or a straight docker install on pretty much any machine that supports either install method since the resources to run HA in either way are pretty much exactly the same.
Heck, I ran both a supervised install and two docker installs simultaneously on the exact same machine for a long time for testing.
And if your docker install is that easy for you then there should be no reason to need add-ons since thry are literally just docker containers modified to run in to the supervised environment of HA. I run all kinds of containers that are available as add-ons and I don’t see any issues at all in my install.