I think you can download an older version here Plex for Android - #510 by PlexAndroidReleaser
e.g.
https://downloads.plex.tv/Kepler/kepler-armeabi-v7a-amazon-10.18.1.1096-e2b6172f.apk ?
Step 1: Disable Auto-Updates for Plex
To prevent the Plex app from updating automatically to the latest version:
- Go to Settings on your Firestick.
- Select Applications.
- Choose Appstore.
- Turn off Automatic Updates.
Step 2: Enable Apps from Unknown Sources
- Turn on your Amazon Firestick.
- Navigate to Settings from the home screen.
- Go to My Fire TV or Device.
- Select Developer Options.
- Turn on Apps from Unknown Sources.
- If you see ADB Debugging, turn it on as well.
Step 3: Uninstall the Current Plex App
- Go to the Settings menu on your Firestick.
- Select Applications.
- Choose Manage Installed Applications.
- Find Plex in the list and select it.
- Click on Uninstall.
Step 4: Install the Downloader App on Firestick
- From the Firestick home screen, go to the search icon and type “Downloader”.
- Select the Downloader app from the search results and install it.
Step 5: Download and Install the Older Plex APK
- Open the Downloader app.
- In the Downloader app, enter the apk download URL (tip: use a shortening URL website like https://t.ly)
- The Downloader app will download the APK file.
- Once the download is complete, the app will prompt you to install the APK.
- Click on Install.