#maintenance mode

No I think we should add a CR for two reasons:

1. If someone has replaced or removed the event subscriber in custom code to try to fix this behaviour without a core patch (unlikely but not unheard of) it's a clear indication that workaround can be removed.

2. It's a change of visitor facing behaviour that's been like this for a very long time. I very much doubt anyone relies on it, but again a CR doesn't hurt.

In that case I assume we don't need a CR anymore, which I believe was the only outstanding item.

The bc policy isn't very clear here, going to try to update it (relevant bit is here: https://www.drupal.org/about/core/policies/core-change-policies/bc-polic... → )

IMO when we're completely removing an event subscriber, we should just remove the whole thing without deprecation (as we would with a hook implementation).

I'm not sure how to deprecate an event subscriber since https://www.drupal.org/node/3357408 → .
It seems there is no way to prevent the event subscriber from handling events without removing it entirely (or disabling autowiring for the entire module).