As discussed on Slack, repositories should be split based on languages and/or platform.
This is so that each plugin can have its own development cycle.

For the "code" part, I found several methods:

• GitHub Docs - Splitting a subfolder out into a new repository
• How to split a git repository into two different ones
• Splitting Your Git Repo While Maintaining Commit History
• Git — How to Split Git SubDirectory into Separate Repository?
• Difference between git filter-branch subdirectory-filter & git subtree split -P

To migrate issues, I found this:

• GitHub Docs - Transferring an issue to another repository