It sure could be a bit of a unicorn. I don't want to speculate on here, but I think there's really little need for such a product. The Quaker State SN rated HM 10W-40 was, but why would someone need a high mileage oil in a rating that has only just become recommemded by some OEMs? By definition, it's for older cars.
Mobil hasn't updated their chart since late last year, but I would expect they will before long. There were five +800 ppm phosphorus SN oils listed on this version, but I don't think any of them needed to be, as they met other ratings for which the oils were primarily intended. Diesel, racing, European. It will be interesting to see what they do. I think few majors who publish their ZDDP data like this.
Mobil 1™ FS 0W-40 oil already contains a higher level of ZDDP (1,000 ppm) that could benefit your flat tappet engine.
www.mobil.com
Valvoline is showing a tech data sheet with SP rated high mileage oils in 10W-40 and 20W-50, but only 750 ppm S.
sharena21.springcm.com
What's it all mean? Not much I suppose, but I find it a little bit interesting. Thanks for indulging me.