I thought this idea after seeing a discussion on Reddit (https://www.reddit.com/r/Music/comments/uzktg0/albums_that_marked_the_end_of_an_era/) about albums that marked the end of an era (https://tvtropes.org/pmwiki/pmwiki.php/Main/EndOfAnAge) in one way or another.

I'll give you some examples:

Modern Family's conclusion in 2020, according to the linked article (https://medium.com/luwd-media/moments-of-modernity-the-end-of-modern-family-marks-the-close-of-the-obama-era-of-tv-optimism-6e6c14c05b43), marked the close of the Obama era of TV optimism.

When Orange is the New Black (https://www.washingtonpost.com/arts-entertainment/2018/10/18/orange-is-new-black-cancellation-marks-end-era-that-forever-changed-tv/) came to an end in 2019, it marked the end (https://www.telegram.com/story/entertainment/local/2018/10/20/orange-is-new-black-cancellation-marks-end-of-era-that-forever-changed-tv/9503702007/) of the first wave of in-house Netflix programming that was aimed at adults (which also included Hemlock Grove and House of Cards).

Speaking of Netflix, according to this article (https://www.wired.com/story/1899-cancelation-future-of-netflix/), the cancellation of 1899 marked the end of its era where weirder, more obscure shows were given space—and time—to thrive. With that, 1899's cancellation also showed that Netflix, like any streamer, was now in the position of having to operate much like the TV networks that came before it.

The 2019 finale of Game of Thrones according to one critic (https://www.washingtontimes.com/news/2019/may/16/game-thrones-finale-marks-potential-end-era-script/), may have marked the end an era of scripted TV shows capturing a wide audience.

2019 in general, arguably marked the end (https://www.theringer.com/tv/2019/12/4/20994172/year-in-television-2019-game-of-thrones-disney-plus) of the third (https://en.wikipedia.org/wiki/Golden_Age_of_Television_(2000s%E2%80%93present)) golden age of television (https://www.pastemagazine.com/tv/the-golden-age-of-television/the-golden-age-of-television-is-officially-over/). Although, it's also arguable (https://theconversation.com/better-call-sauls-final-episode-is-the-end-of-the-golden-age-of-tv-as-we-know-it-188447) that the 2022 conclusion of Better Call Saul was really wen this particular era ended.

Cartoon Network's Ed, Edd n' Eddy (https://collider.com/ed-edd-n-eddy-cartoon-network-end-to-an-era-explained/) was the last big, Western animated series to utilize the old school hand painted method instead of the soon to be industry standard, digital ink and paint.

The 2023 cancelations of Doom Patrol and Titans arguably marked the end (https://www.inverse.com/entertainment/doom-patrol-titans-cancellation-end-of-dc-tv-era) of DC's best era of live-action television.

When ER ended in 2009, it arguably marked the end (http://www.cnn.com/2009/SHOWBIZ/TV/03/31/er.finale.drama/index.html) of costly, high quality drams on network television. By this time, "prestige" television (https://www.theringer.com/tv/2019/9/19/20872849/er-how-it-was-made-20th-anniversary) shows could be found more and more on cable.

The year 2010 according to this article (https://www.cartoonbrew.com/ideas-commentary/the-end-of-the-creator-driven-era-29614.html), marked the end of an era in creator driven animation.

The series finale of Royal Pains in and the debut of Mr. Robot in 2016 officially marked the end (https://www.washingtonpost.com/news/arts-and-entertainment/wp/2016/07/07/farewell-royal-pains-and-hello-mr-robot-usas-era-of-blue-sky-shows-is-over/) of the USA Network's "Blue Sky Era" (https://www.cbr.com/usa-network-blue-sky-era-better-remember/).

According to AVClub.com (https://www.avclub.com/the-last-barney-miller-also-ended-an-era-1798276904), when Barney Miller aired its series finale in 1982, it pretty much indicated that the ’70s were over, and the ’80s had begun. Shortly thereafter, Cheers and Family Ties would debut.

I would mark at around half of the 2000's the end of the sitcom era, with the end of Frasier, Friends and partially Will & Grace.
Of course the genre still had a few hits but lost his crown as the main genre. Most of the comedies are now single cameras with no laughs track nor live audience, and dramas took the edge as a genre.

Since I mentioned Barney Miller, 1982-84 seems sort of like the last real "hurrah" of the big '70s sitcoms. In that time frame (http://www.thetvratingsguide.com/2018/01/1982-83-sitcom-scorecard-cheers-is.html), M*A*S*H, One Day at a Time, Archie Bunker's Place (and by extension, All in the Family), Laverne & Shirley, Happy Days (http://www.thetvratingsguide.com/2017/09/1983-84-sitcom-scorecards-no-more-happy.html), Taxi, and Three's Company all met (https://content.time.com/time/subscriber/article/0,33009,951165,00.html) their conclusions. And come 1984-85, The Cosby Show would debut.

One comment (https://jacksonupperco.com/2016/05/24/the-ten-best-night-court-episodes-of-season-two/) that I read when discussing Night Court (which debuted as a midseason replacement in the 1983-84 season) was that the 1984-85 season is the first where TV really embraced the '80s as a decade with a style and comedic sensibilities — for better or worse — that are entirely separate from the ’70s (https://www.reddit.com/r/television/comments/lb9u9k/what_caused_the_difference_between_70s_sitcoms/). It was an aesthetic space to which many shows of the transitional '80-'84 period were more attuned. And with these '80s sensibilities comes a tamping down of the cynical grit that defined many ensemble shows (such as Barney Miller) of the decade prior.

I'll go with the end of the third season of Laugh-in. That was the last year having the bulk of the core cast. From season 4 -6 it seemed to be a bad imitation of it's self. Especially season 6. They ran out of gas when season 4 ended. And season 4 wasn't as good as the first three.

As for the end of an era. The first 3 seasons of Laugh-in bought in nicely to the late 60’s pop culture and the questioning of the establishment. See Flying Fickle Finger of Fate Award for questioning the establishment.