I don't know how close, but SQL Server 2019 must be almost done. If you have been paying attention, you will notice that CTP 3.0 was released this past week. There are a few changes, but this note is the big one for me: "For customers in the Early Adoption Program, CTP 3.0 is the first release where you’re able to run SQL Server 2019 in production." This is reflected in the release notes as well. The notes also reflect that in-place upgrades of SQL Server 2012-207 are now supported. No upgrades from other CTPs, but that's expected.
It seems a lot of work has been done, and some problems have been fixed. Notably, a new compiler was used with CTP 2.1 that had some issues with conversion scenarios. Those appear to have been cleaned up. There are limitations with UTF-8, as well as some issues with Secure Enclaves. I can't imagine the platform will RTM without Secure Enclaves, but what do I know. SQL Server 2005 was released without Database Mirroring working, with the issues ironed out in SP1 shortly after release.
The relatively few items mentioned as changes would indicate to me that much of the feature work is done and now tuning and bug fixes are under way. I wouldn't be surprised to see another CTP or two, but I do think that we may see a final RTM sometime this summer. I've seen a lot of guesses in the forum about the release date and some of them seem better today than they did last month. My own guess, with no inside information, is that we will get 3.1 and 3.2 as CTPs before an RTM in August.