haskell-stm is included with ghc and won't be shipped as an installable package in Debian buster.
Propellor needs to stop trying to install haskell-stm on Debian buster or newer (and it should also be dropped from debian/control of course, but that can happen on the Debian side).
--spwhitton
Bootstrap's package installation ignores errors, so if the package is not available, it will at worst display an error message and continue.
However, I went ahead and made it check with apt-cache if the package is available, to avoid the user seeing an apt error here.
I suppose users of Arch and FreeBSD may want to make similar changes, if their package managers also display errors; I don't know if they do or even if this change is going to affect those distributions. Probably better to let those users deal with it if similar changes are made to those distributions, so I'm going to close this.