I think it's more complicated than simply the window getting smaller.
I mean, first, there are contemporary FRPGs which include slavery in their rules. Two I know of are Burning Wheel and Torchbearer 2e. The former has a series of Servitude and Captive lifepaths, that include Born Slave. They are there in my 2005 Revised Edition, and still there in my 2019 Gold Edition Revised. In tone/theme these lifepaths combine ideas of pre-modern European slavery with pulp/S&S. Torchbearer 2E includes the following sub-section in its discussion of establishing a PC base (Lore Master's Manual p 178):
Say What Now?
*Did we say that these rules afford you the opportunity to take captives and work them to death? Yes, and we would like to acknowledge that taking captives and working them to death for your own selfish aims is the penultimate of evil, second only to torturing and murdering living beings.
*Keeping living beings captive against their will is also evil, and exploiting their free labor is as well. The only element in favor of this act is that they are alive and may one day rise up, throw off their chains and destroy their enslavers.
*So why allow these awful acts in our fantasy roleplaying game centered around robbing and murdering? Because there can be no light without darkness. In this game, whether you live in darkness or light is your choice.
There are probably examples, too, from other contemporary RPGs.
A second complication is the one mentioned by
@Irlo. Characters and situations that one didn't see in FRPGing 40 years ago, now do appear. So in that sense the window has opened wider.
Burning Wheel is an interesting example in this respect: from 2005 to 2019 it has included a lifepath that represents an openly gay man in a default conservative (pseudo-)mediaeval society. But between 2011 Gold Edition and 2019 Gold Edition Revised, a key feature of the lifepath was changed, so that it no longer includes a mandatory trait to represent the social controversy of being openly gay. One might say that this changes the shape of the window, but I don't think it is obviously a reduction in its size.