If you simply navigate too https://downloads.openwrt.org, the release is not shown until it is announced. However, the build bots save the results onto the download server's "releases" directory which is just simply served up as a directory listing, so anyone can browse the contents there -- and people do check that directory, especially as new releases approach.
In writing my warning in the OP, I am not trying to "solve" the "problem" of users installing unannounced releases... it is to ensure that they understand the potential risks or quirks of the system. Every time a new release is in the build phase, people inevitably complain that the image for their device cannot be found, or they do manage to install and then say that packages are missing. There is a reason there is an official announcement made when each release is finished building and verified... so those who don't want to wait may encounter some issues.