phpPgAdmin is not available for installation in Tools & Settings > Updates and Upgrades > Add/Remove Components page.
There is no phpPgAdmin button to log into PostgreSQL instance at the Tools & Settings > Database Servers page.
phpPgAdmin support was dropped starting from Plesk Onyx 17.8
Vote for this feature request by the following link: Include pgadmin for Postgres management in Plesk.
The top-ranked suggestions are likely to be included in the next versions of Plesk.
As for now, customers who need to edit their database using a web interface can install a stand-alone phpPgAdmin on their websites.
Warning: manual installation of phpPgAdmin is not be supported by Plesk and performed at your own risk.