Just trying to address the “general question”, the different build types are described here Different Build Types.
Think of each build type having its own release channel. In general users are not going to change release channel lightly.
Again, in general, once a device is successfully Upgraded in Android version, in a certain release channel, there is no real need to keep supporting the “old” Android version.
Edit, in light of the reply by @hirntot, thanks, conversely also where there is development work happening in more than one Android version, then we can expect /e/ ROMs to be published at more than one Android version.