I believe that the original reason of the removal of certain plugins was a technical limitation.
They went from the app plugin's menu, then to the playstore, then Android 10 introduced scoped storage so they came back to the the app...
There are ways to install it (or them):
(Here it will be !addplugin apkbuilder)
But even if you'll be able to install all of them, they'll only work if you purchased them in the past (or if you are/go premium).
And there is actually no possibility to buy official plugins independently.
My guess is that the purchase method has been (temporarily) unavailable due to the changes made to install the plugins, but I also suspect that it has finally been abandoned against the premium subscription (probably more convenient to implement and more lucrative on the long term).
So for the moment, to "purchage" apkbuilder, premium is the way to go.
If you already bought it in the past you can install it again and use it right away (you have to use the account which was used for the purchase).
But even here you'll be stucked with apks and won't be able to build aabs:
To conclude I'd say that even if I understand the need to have a regular income to make the project live, new users (especially young ones) won't go premium.
And being unable to create an APK out of DS projects can be off-putting for many new and casual users.
I hope it will change in the future