Are there any in-depth articles or discussion threads on when and how the phenomenon of the "season finale" (where the show tries to end with a bang (https://tvtropes.org/pmwiki/pmwiki.php/WhamEpisode/LiveActionTV) of some kind) developed?

It seemed like in the earlier days of TV, sometimes a TV season would just peter out ignominiously with a backdoor pilot (https://tvtropes.org/pmwiki/pmwiki.php/PoorlyDisguisedPilot/LiveActionTV) for another show (Star Trek TOS' "Assignment: Earth" at the end of Season 2, for instance) or a clip show (https://tvtropes.org/pmwiki/pmwiki.php/Main/ClipShow).

Did this change mostly start with Dallas' "Who Shot JR?" in 1980 and then accelerate in 1990 with Star Trek: TNG's "Best of Both Worlds, Part I"?

Good question!!! Interesting topic!!!

Yeah I'd say the whole "who shot J.R." storyline was the first (or one of the first) big season finales for a TV show. And eventually more shows started doing them in the 80s and it's just gotten more common since then. If there were any shows that did a big season finale before 1980 I'm not aware of them. But it would be interesting to know if anybody else remembers anything like that on a TV series before 1980.

Ed.

Not a cliffhanger or anything but Little House on the Praire " I'll Be Waving as You Drive Away" when Mary goes blind was kind of big. I remember the whole show startled me mainly because I didn't know the backstory of the real Mary Ingalls. That was in March of 1978.

“Dallas” really started in the modern TV era. Of course, the other soaps followed. I think “Cheers” was one of the first sitcoms to follow suit.