Something similar did happen in the past: As part of OpenTelemetry's
move from sandbox to incubation, the OpenTracing project agreed to
archive itself.

*Without having looked at the details, going just from what you described*:
While the project is maintained, even without substantial feature
work, it seems that no immediate action on project status is needed.
Personally, I would put deprecation notices and pointers to migration
guides in all the right places to avoid onboarding new users and
making offboarding existing ones more effective and efficient. Once
you see that a sunset is feasible send a timeline to your last users
and share it with TOC.

NB: It seems unlikely based on what you described, but the approach
above allows interested third parties to step up to become Contour
maintainers if they want to carry the project forward long-term.