Consider what happens if the default option of "mark these projects as in maintenance mode only and archive the repos" happens.

The maintainer has not said anything about archiving the Github repositories. Archiving the repositories on Github is not compatible with keeping the project in maintenance mode. Github does not allow modifications to an archived project.

Maintenance mode is not necessarily bad thing. The packages are mature.

How would most users of these tools find out?

If the maintainer does abandon the packages (and there's no discussion of this!!!!), the maintainer can inform dependencies through a deprecation comment in the go.mod file.

What would succession look like? What would it look like in the ideal case?

The maintainer describes the succession plan in the Github issue and comments.