I have re-thought this a little.
Instead I will move ST2 to branches named ST2 in their respective repos. I will not add new features to them, nor will I actively support them (though I am free to make exceptions if/when I choose).
Bug fixes and features will only be applied to the ST3 branches. All pull requests will need to be accepted on the main branch. I may accept small requests for settings updates like BracketHighlighter or ApplySyntax rule definitions, but I will require users to also pull request relevant ST3 related changes to the main branch in order to get ST2 pulled in. This will take the extra work of me remember to back port crap to the ST3 branch when people only pull the ST2.
This will free up the main branch for development. When ST4 comes around, if needed, I can simply fork the main to ST4.
PackageControl pull request has already been made for this change. After that ST3 changes will all be merged to the main branch in their respective repos, and they will become the dev branch.