I want to suggest a new step

Which tool is this about? Where is its repository?

I would like to see support for LURE, it is trying to be an alternative to the AUR but with the goal to be distro agnostic. So far it is looking like a very interesting project, and I am successfully using a few packages. The official repository is on the developer's Gitea, but there is also a mirror on GitHub.

Which operating systems are supported by this tool?

All Linux distros.

What should Topgrade do to figure out if the tool needs to be invoked?

I think by default the binary is at /usr/bin/lure, so maybe begin with checking that?

Which exact commands should Topgrade run?

Just running lure up should be enough. Maybe also lure --interactive=false up in case the default yes flag is set for Topgrade.