Here is the most used fork for anyone intrested and a few thoughts on other tools:

I tried the mentioned Whats up docker and Diun (docker image notifier), but was not satisfied with both.

Nowadays I use Komodo, it uses basicly docker compose with a very nice web UI and it can do autopolling, notification or auto update itself. It is a very powerful tool and even replaces Portainer.

The announcement is thoughful. It explicitely tells user the tool won't reveive maintenance so they know to look for alternatives right know.

Silently abandonning a project could cause an accumulation of issues, unmerged MRs over time until peole realize the maintainers aren't coming back.

Nextcloud has decided to use the following fork, which seemed good enough for me as well: https://github.com/nicholas-fedor/watchtower