Most users of tiles.wmflabs.org would be better suited to the production tile server.
If people want to stick with tiles.wmflabs.org, they can - it's not a WMF maintained resource, so it's no additional development work for any teams, but I'd hate to see people using it because they didn't realize they could substitute it.
I'm not sure who to ask for a list of users. This should be in server logs somewhere in the form of referrers.
If people are wanting to stick with it, it'd be good to know why and what they're getting from the old osm-carto style it uses that the current WMF style doesn't do.