Why does a thread have a red box around it?

[SootyandMathew]

Just went to the list of AIBU threads and one has a red box around it saying it was deleted.

Is this a new thing or is it because I'm on my phone ?

I can't see anything red boxed on AIBU list (am on laptop) - which thread was it?

I think the red box happens immediately after hq delete the thread , I’ve seen it before, it’s not new as far as I know.