We finally did it, thank you for all your help and feedback.
https://nim-lang.org/blog/2018/09/26/version-0190-released.html
The fact that nil is not a thing anymore for seqs and strings is a large breaking change so please update your Nim-code / Nimble packages. That said, thanks to the switches --nilseqs:on --laxStrings:on the transition period is rather smooth IME.
Nim 0.19.0 is in Debian Unstable since yesterday evening. Derivative distributions will fetch the package in the next days.
A little dashboard: https://repology.org/metapackage/nim/versions