I then check about:config and dom.private-attribution.submission.enabled is still set to true.
Looking at the Mozilla wiki (https://wiki.mozilla.org/Privacy/Privacy_Task_Force/firefox_...) I don't see any discussion of telemetry as controlling this behaviour.
It is not clear to me what is going on or what the state of this feature actually is. Have I opted out or haven't I?
After the initial complaints they must have known it was controversial. I stand by my original point, they could have been much more upfront about this.
The setting being greyed out indicates that the entire functionality is disabled, and the value of the setting is irrelevant. That's a common pattern in Firefox settings when some other setting invalidates the current one. In this case, it's most likely because you have "Allow Firefox to send technical and interaction data to Mozilla" unchecked.
https://wiki.openstreetmap.org/wiki/Tag:roundabout%3Dturbo
https://en.wikipedia.org/wiki/Roundabout#Turbo_roundabouts
https://www.arcadis.com/en-us/knowledge-hub/blog/united-stat...