Craft 3 plugin backwards compatibility and maintenance for 2.x
This seems to be the common pattern emerging amongst developers in the same position...

A single repo:

craft-(pluginname)


Containing multiple branches:

v1 (your old Craft 2 version)
v2 (your new Craft 3 version)


There are a handful of variations to this, but that's it in a nutshell.

Examples:


https://github.com/craftcms/contact-form *
https://github.com/barrelstrength/craft-sprout-notes
https://github.com/doublesecretagency/craft-cpcss
https://github.com/marionnewlevant/craft-snitch
https://github.com/carlcs/craft-deleteentryversions


* P&amp;T has mentioned that they didn't prefix their own repos with craft-, since the repo is owned by craftcms. But they said they would have, if they were a 3rd party developer.

Some people have prefixed their repos with craft3-, but that has been discouraged by the folks at P&amp;T. It would force you to abandon this repo (or leave the name confusing) when Craft 4 is eventually released.