Groups Lifecycle / Archiving / Moving

As has been pointed out, there are many complications to address. For us, the ability to move the document library to a SPO library archive - we use a document center site - from Groups would be a big help.

We've been using team site (and project sites) for years and would love to use Groups instead, For that to truly work for us we'd need to be able to create our own Group document library template that makes use of our custom content types which also requires access to our metadata store. Our use case would be - create a group for the project, inviting all relevant internal and external users and then archiving the documents (by moving to SPO as above) and perhaps email.

Email isn't as huge an issue since we set it up so that group members get emails from the group so the messages can be archived that way. 

We're not subject to regulatory compliance so we don't face those issues.

Of course, the easy way is to do nothing - keep the group and allow users to either leave or just remove the group from their Outlook clients and perhaps hide them in OneDrive. that will get messy after a while of course.