https://emacs.stackexchange.com/questions/17063/how-to-setup-knitr-workflow-in-emacs recommends version 19.04 as the setup for Knitr is much simplified. However I can't find version 19.04
http://ess.r-project.org/ has version 18.10.2 and links to a development version, 18.10.3snapshot, on github. It describes version 19.04 as "unreleased."
I'm going to add my own answer to this, as I was deeply confused.
As far as I can tell, in a technical sense "ESS 19.04" does not exist - when people refer to ESS 19.04 they mean "the current development version of ESS", which lives here. The VERSION file there currently (September 2022) lists 18.10.3snapshot; the version number hasn't changed since 2018 even though there's been a lot of development since then.
The news file on the ESS page discusses 19.04 extensively, and lots of web resources refer to version 19.04 ... but good luck finding it!