hub.docker.com no longer seems to have an image for tt-rss, not even a trace of the page for it. has it been dropped?
Yes we depreciated this image see here:
Damn. Well, thanks for the support you did give it. Back to bare metal install for me.
Can you update the Dockerfile to download the master tarball instead of a specific version?
We are no longer able to ingest tarballs from upstream repo
What was the reason for the deprecation though? I don’t understand why this is an issue?
Upstream dev wasn’t a fan, we prefer to avoid upsetting people. 
I can understand your frustration, but that is all the information.
If you would like to continue using this image feel free to build from source which is still available on GitHub:
The same statement goes for any potential maintainer , feel free to build and publish this image (the dev branch consumes their master ) we will no longer be supporting it.