The setting was removed from Plex Web in PMS v1.19.3.2740, released in April.
The default location is %LOCALAPPDATA%\Plex Media Server, which is usually C:\Users\<username>\AppData\Local\Plex Media Server.
https://support.plex.tv/articles/202915258-where-is-the-plex-media-server-data-directory-located/
