Kodi has just announced that as of Kodi 19, Python 2 support will be discontinued. This comes as no shock, since Python 2.7 will not be maintained at all past 2020. Python 3 was released about ten years ago, and was ported to Kodi thanks to a Google Summer of Code project this past summer. Team Kodi’s current plan is to implement their Python 3.6 interpreter in Kodi 19, so not the next major release version, but the one after that. Unfortunately for addon developers, Python 3 is not backward-compatible with the the current version.
The good news is that Kodi 19 doesn’t even have a name yet, so you have time. At some point within the next two years though, you’ll have to convert your addons to Python 3 compatibility in order for them to function. According to the official Kodi team, it isn’t very difficult to write code that’s compatible with both Python 2 and Python 3. They’ll also be releasing test builds of Kodi 18 that include Python 3 shortly, in order to make testing more straight forward.
Lastly, Team Kodi stated that this is their preliminary plan and may be subject to change.