Photo libraries are still supported by your Plex Media Server and there’s a dedicated Plex Photos app for the mobile platforms.
https://www.plex.tv/media-server-downloads/?cat=mobile&plat=ios#plex-plexphotos
The removal / initially missing support for photo libraries in the „new experience“ iOS app was mentioned in the release notes.
There’s also been an announcement in Plex‘s Blog (and iirc an in-app announcement, though you won’t get those if you’ve disabled notifications).
https://www.plex.tv/blog/the-future-of-plex-focused-streamlined-and-ready-for-feedback/
As pointed out by others, Plex have since then also announced that support for photo (and music) libraries will be added back to the main apps.
