The Maps team would ideally like to upgrade PostGIS to v3.1 since I believe it comes with performance improvements. This is part of the Maps stack upgrade work some of which is already underway and parts of which have been completed. We prefer that the new stack run on buster, not stretch, so we can start off without needing to worry about backports, etc.
The reason behind the PostGIS version is some performance improvement we would like to use in our spatial queries ( asMVT queries and feature IDs ).
According to the PostGIS docs, PostGIS 3.1 is compatible with Postgres 11 which is already available for debian buster.
Filing this task for the service-ops team to explore the effort involved in packaging this. On IRC, @MoritzMuehlenhoff said that PostGIS 3.1 is already packaged in debian official.