Plesk Onyx 17.8 is installed.
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 is deprecated in Plesk 17.8: it is removed from existing installations during the upgrade to Plesk 17.8 and not available for installation via Plesk anymore.
This has been done due to security reasons: phpPgAdmin project has been inactive for the past 3 years, which lead to phpPgAdmin becoming an insecure and unstable application.
This feature is not yet implemented in Plesk, 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.