Though we won't be able to actually do any uninstalling -- because we need the module to do that -- we can at least let the user know that something needs to be uninstalled.
If there is an entry in the file system for a module that is disabled (or enabled but removed) I'd like to show information on the Uninstall modules page prompting the user to add back the missing code, and uninstall properly.
It could be tricky to determine which modules need to be uninstalled (and which don't) but I think it's a worthwhile effort to think about how we might do that.
We could even (possibly?) use the schema_version to determine which version of the module to recommend.
This was spun-off from https://github.com/backdrop/backdrop-issues/issues/541
Recent comments
You can use Rules or the Node Notify module for comments. When a new account is created, I believe there are Rules to send.
Email notifications
There's a few way to accomplish it, but it really depends on your full requirements. I'm working on a contact management module but it won't be released for a few months. There...
Pairing fields
I'm fairly sure that some people have done this successfully in Backdrop. Please can you list the modules you are using; that should help us get the right people to join this conversation.
How to render facet blocks?