While not an 100% answer to the exact question, it seems that at least PostGIS functions can be "grouped" in a schema of my choice if it's at least PostGIS 2.3:
https://postgis.net/2017/11/07/tip-move-postgis-schema/
The main points of the above link:
CREATE SCHEMA postgis;
ALTER DATABASE mydb
SET search_path = public,postgis;
UPDATE pg_extension
SET extrelocatable = TRUE
WHERE extname = 'postgis';
ALTER EXTENSION postgis
SET SCHEMA postgis;
ALTER EXTENSION postgis
UPDATE TO "2.4.1next";
ALTER EXTENSION postgis
UPDATE TO "2.4.1";
This will move all PostGIS functions to a scheme named "PostGIS", that pgadmin can expand/collapse.