Anchor text – 5.9.1 update – Where is the auto option?

In 5.9.1 the changelog states that auto-generated heading anchor text has been moved to opt-in. In 5.9 it was on by default.

I like this feature, and with 5.9.1 it has been turned off, but I can’t find anywhere to turn it on. The GitHub notes for this change state it can be turned on, but don’t shed any light on where you might do this. I have looked in block editor preferences, I have looked in Settings>writing, I have looked at options when a heading block is selected… nothing.

Has anyone found this setting?