The backport is non-trivial, and NixOS 19.03 is supported until the end of October.
https://community.letsencrypt.org/t/end-of-life-plan-for-acmev1/88430/7 says account creations will still be possible until around November 8 (affecting newly added domains), and renewals over the old API (affecting existing domain’s renewals) should still supported until June 2020. So this doesn’t break anything in 19.03 during its support window - I’d recommend upgrading to 19.09 anyways
If you really can’t upgrade and a backport isn’t going to happen you might be stuck switching to an imperative certbot command. Not ideal, but sometimes that’s what happens in a pinch. Good luck!