Any chance of supporting Unity 2018.4? I asked about that in another thread, and you mentioned that you would support up to Unity 2018.4, just double-checking!
2018.4 is important because it's the LTS (Long Term Support) release, and Unity will be properly supporting it with bug fixes, etc. for nearly two more years, which will give a lot of time to move over to the built-in Unity 2D.
Looking at how Unity 2017.4 has been handled for the last few years, there weren't any major changes, just bug fixes, support for necessary things like 64-bit on Android, etc. (none of which would affect 2D Toolkit, probably).