While trying to work out fill-in flash semantics, it struck me that for the purpose of "overpowering the sun", a short flash duration is rather important since it allows you to open aperture up while decreasing exposure time, keeping the overall brightness while increasing the effect of the flash.
It turns out that my ancient (from the 80s I think) Regula Variant flash specifies 1/1000s flash duration at full power (for which it has guide number 40 at its native f=35mm light angle and up to guide number 70 at f=200mm when using a separate "tele lens" attachment with a fresnel lens).
Going through a list of Metz flashes of similar age and power (including wand flashes with about double the power output), the full power flash duration pretty uniformly ends up as 1/200s. Requiring 5 times the exposure time makes shadow lifting at a distance quite more tricky. Particularly with fast leaf or electronic shutters, it significantly impacts the efficiency of a flash to deal with competing-light situations.
A current-time Godox TT685 has a guide number of 60m at f=200mm (if we consider the Regula specs a bit optimistic, that may be comparable) and a specified duration of 1/300s.
So what gives with regard to the large difference in specification? It cannot be the switch from thyristor technology to IGBT since the older Metz flashes still use thyristors as well. Is the flash bulb different possibly (the size factor seems the same as comparable cobra head flashes today) or driven outside of its comfortable specs?
In analog camera times, the utility in "overpowering light" situations would have been more limited due to larger flash sync speeds, so the main utility of such specs would seem to have been motion freeze in the dark. With modern flashes (and modern superzooms), large reach at short flash time would seem at least as important as it had been at old times.
Why were those kind of specs generally given up on for consumer-level on-camera flashes?