My content is not vintage but 'tweener years'.
For vintage sloping = read - Mixte. and all the step-thru derivations.
Around '98 I heard the term - upslope. Slight rise in the top tube.
Not sure when Giant or someone else first did the CF compact frame design w/ mtb seatposts of carbon.
Upslope.
Perhaps a bit more standover?
Don't know if it was for a taller head tube?
Certainly not to stiffen the rear triangle.
On my custom steel bike, ordered in 2003 (delivered 2004), we targeted a taller head tube and achieved it with a couple of cm of upslope and slight extension, without raising the standover by a similar amount. Subtlety in design to solve a given concern.
Frame by Jon Tallerico - central California. Mixed Columbus tubing, lugged construction with mods for 1 1/8 headtube, Columbus SuperMuscle fork, Campy10 mixed, FSA compact crank, Deda stem & bars. Now running tubular wheelset and different saddle.
Any way, i've not seen vintage production drop handlebar, classic steel
lightweight bicycles with sloping geometry. Middleweights - all bets are off. Aluminium frames got shaped and sloped for aero and stiffness reasons, but i also heard it said that increasing the welding perimeter made Al frames more dependable (stronger?).