If you were in this situation yourself, what would you do if you saw a test failure? Is it different from what you'd do if you found it to be a success?

If you put it into a tier of support that states "I won't be proactively maintaining compatibility and fixing issues here", then your answer is "nothing." A failing test won't spur you to action, because of the support level.

So if the test result doesn't change your behavior, why run the test in the first place?

This is very naive.

CI jobs cost money, and every new target has a multiplicative effect on the number of job configurations that need running.

It's quite a lot of extra effort.

The Rust test job is to compile every single public open-source package on the crates.io registry. This is an amazing way to test. But it's also expensive, because that actually takes a lot of machine time.