Missing Python build

Hello, apologies if this is the wrong forum for this question.

We wanted to check if there are any current known vulnerabilities in a particular tag, specifically python:3.6.1-slim.

However, I couldn’t see that image in the list here: https://hub.docker.com/r/library/python/tags/

The nearest image I could find is the one for python:3.6.3-slim; going to what would presumably have been the URI for the tag we’re interested in is a 404: https://hub.docker.com/r/library/python/tags/3.6.1-slim/

Any idea why this tag would have gone missing / what have we misunderstood about the process by which tags are monitored and obsoleted?

thanks in advance