Unless you are a paying Alfresco customer, the officially supported list is nothing more than a suggestion anyway, as you won't get any immediate support from Alfresco anyway.
Since the documentation on docs.alfresco.com has recently received a large overhaul, there may be some discrepencies between historic information (in the GitHub archive you found) and what you may find elsewhere. Currently, the olderst version for which you can get the supported platforms is 6.0: https://docs.alfresco.com/content-services/6.0/support/
Alfresco generally does not change / update the versions of supported databases and other tools within a release (e.g. 5.2 vs. 5.2.1, 5.2.2, and so on) unless there have been significant reasons, i.e. deprecations of versions or specific support issues.
With regards to databases like PostgreSQL, you can generally use anything that is the same or a newer version than the one(s) listed. Alfresco will only list additional / multiple versions if they specifically did separate evaluation runs with those versions.
So, to answer the question in the topic: Alfresco has always been compatible with Postgres 10 and 11 - it just may not have been tested before / inititally, so the documentation may have been updated afterwards.