This is inspired by an innocnt user asking on r/kpophelp if any idol spoke "Seoul dialect". What they meant was of course the "standard language" a.k.a 표준어, and given that in many capital cities of the world they speak the standard language of that country's language, I get the OP's thought process.

But Seoul dialect exists, and it's definitely not standard Korean! The 서울사투리 is a dialect or accent that was very widespread in Seoul during the 60s to the 90s, and is a "derivative" of the neighbouring region Gyeonggi-do's dialect. As it experienced a decline in the mid-to-late 90s, no person under 30 except maybe one or two use the accent anymore, and it's seen as a "older people" thing to speak in it. It's seen as a "posh" dialect, as opposed to the "rough" dialect of the provinces. Some specialities of the Seoul dialect include :

(I'm going to use a simple sentence as an example : 우리 엄마는 이번 방학때 매일 회사 다녔을 때 소연 이모와 혜선 이모와 지영 이모와 퇴근하고 노래방에 가고 놀다가 늦게 집에 왔어요. (trans: "This holiday, when my mother went to work everyday, she left work with Auntie Soyeon, Auntie Hyesun, and Auntie Jiyeong, and came home late after going to the karaoke"). Try reading along if you can read hangul!

- frequent use of the 시- form in formal speech : 우리 엄마는 이번 방학때 매일 회사 다니셨을 때 소연 이모하고 혜선 이모하고 지영 이모하고 퇴근하셨고 노래방에 가고 놀다가 늦게 집에 오셨어요

- (over)frequent use of -요/-여 at basically the end of every sentence part that they want to highlight (and if the sentence isn't finished, add a -고), and raising the tone at said end significantly : 우리 엄마는요, 이번 방학때는요, 매일 회사 다니셨을 때 소연 이모하고 혜선 이모하고 지영 이모하고 퇴근하셨고요, 노래방에 가고 놀다가 늦게 집에 오셨어요.

- sometimes , but not always : usage of ㅐ instead of ㅏ , and ㅜ instead of ㅗ, and ㅕ instead of ㅛ : 우리 엄마는여, 이번 방학때는여, 매일 회사 다니셨을 때 소연 이모하구, 혜선 이모하고 지영 이모하구 퇴근하셨구여, 노래방에 가고 놀다가 늦게 집에 오셨어요.

- pronouncing the 으 as 우 : 우리 엄마는여, 이번 방학때는여, 매일 회사 다니셨울 때 소연 이모하구, 혜선 이모하고 지영 이모하구 퇴근하셨구여, 노래방에 가고 놀다가 눚게 집에 오셨어요.

- frequent usage of -거든 요 and -고 요 as a verb ending, and if it's something that happened to someone else, with a -라고요, which denotes that its someone narrating) : 우리 엄마는여, 이번 방학때는여, 매일 회사 다니셨울 때 소연 이모하구, 혜선 이모하고 지영 이모하구 퇴근하셨다라구여 , 노래방에 가고 놀다가 눚게 집에 오셨거든여 .

- If it's the 70s or 80s, instead of "-을 때" uses "-을 적에" and instead of "거든요" uses "크든요" : 우리 엄마는여, 이번 방학때는여, 매일 회사 다니셨울 적에 소연 이모하구, 혜선 이모하고 지영 이모하구 퇴근하셨다라구여 , 노래방에 가고 놀다가 눚게 집에 오셨크든여.

If you've made it this far, read it again and this time try "softening" the tone of your voice (which gave the dialect it's "posh" vibe). Basically speak like you're trying to comfort a crying child (lol). And tada, you could be transported to 1989 Seoul and wouldn't be out of place (speech-wise).

And yes, there are still celebrities that use the dialect! 99% of them are people that grew up speaking it. There's a bunch of actresses and singers, like Kim Hyeja, Jeon Inhwa, Kang Susie, Ha Soobin, Uhm Junghwa, Yang Joonil, Shin Aera, Lee Youngae, Jeon Jihyun and Sung Yuri that still use it. Yang Joonil even uses an older version of the Seoul Satoori as his parents emigrated to the States in the 70s, and he was born in the States, and learnt their dialect while learning Korean at home. There's a reason that linguists are always on the lookout for gyopos that return as in many cases they still acrry some of their parents authentic accent.

There's actresses that used to use it but have adopted a more "formal" and neutral accent iin modern times, like Kim Hyesoo or Ha Heera. There's also still oldschool announcers that still have it.

As for idols, you'll only see 1st gen idols and singers flaunt it in some of their older interviews, such as Seo Taiji and Boys, S.E.S.. g.o.d. and Fin.K.L., as tey were born in the 80s and grew up with it.

With that said, what's your little detail you want to clear up?