You MUST update to the new package name io.github.ungoogled_software.ungoogled_chromium to keep using Ungoogled Chromium.
The old package name com.github.Eloston.UngoogledChromium is no longer supported and will only display this message from now on.
To update, run the following command in your terminal:
$ flatpak update -y com.github.Eloston.UngoogledChromium
Looking for updates…
Info: app com.github.Eloston.UngoogledChromium branch stable is end-of-life, in favor of io.github.ungoogled_software.ungoogled_chromium branch stable
Updating to rebased version
...
Changes complete.
If you need any assistance or have any questions, feel free to ask in the Ungoogled Chromium Matrix room at #ungoogled-chromium:matrix.org or the GitHub issue tracker.