Soft science fiction may be considered those stories where the science isn’t rigourously probable, starting with Faster Than Light star flight. It is also often considered the scifi using the soft sciences: sociology, psychiatry, economics, political science, anthropology.

Ursula K. Leguin, daughter of anthropologist Alfred Kroeber, can be considered pretty dedicated to this when not doing fantasy: The Word for World Is Forest, The Left Hand of Darkness, Rocannon’s World.

The 1950s saw a lot of this, in predicting social changes, like The Space Merchants or The Clans of the Alphane Moons.

Foundation, Foundation and Empire and Second Foundation by Isaac Asimov, which uses speculation in the ability to analyse and control history.

The Faded Sun: Kesrith, The Faded Sun: Shon’jir, The Faded Sun: Kutath by C.J. Cherryh, which is an exploration of the alien culture and its intersection with others.

Foreigner, Invader and Inheritor by C.J. Cherryh

Engine Summer by John Crowley, exploring changes in sociology in a fractured post-apocalyptic future.

The Man Who Used the Universe by Alan Dean Foster

Friday, Methuselah’s Children by Robert A. Heinlein

The Book of Rack the Healer by Zach Hughes

The Green Millenium by Fritz Leiber

Dragonflight, Dragonquest, The White Dragon, Moreta, All the Weyrs of Pern, Renegades of Pern, The Dolphin Bell by Anne McCaffrey

To Ride Pegasus, The Ship Who Sang, Crystal Singer by Anne McCaffrey

The Dream Snake by Vonda McIntyre

The Beastmaster and Lord of Thunder by Andre Norton

Anthem by Ayn Rand

The Gray Prince, Big Planet and Showboat World by Jack Vance