John
Nothing has changed, the basic design of the strop is as you describe.
Aussie Rob
Plenty of people are rescued using a single strop in the vertical position. It may be that they're hoisted from a position where placing a second strop is either challenging, impossible or would unnecessarily prolong the time required for the helicopter to be in what may be a challenging overhead. Alternatively, there may be obstructions or protrusions (eg from a vessel which is pitching and rolling on the high seas) which make the casualty more vulnerable being winched in the seated position. Furthermore, if there are multiple casualties to be retrieved then you're at least doubling the length of the winching evolution if you hoist each one individually in the seated position.
I hope this satisfactorily answers the 'what would possess' comment regarding the non-use of a crotch strap or second strop.