Is the Modern Seoul dialect losing its length distinction?
Yes, length contrast is being neutralised in Seoul Korean. This is something that has been happening for a while:
- According to Han (1964), Seoul Korean speakers who were back then in their 20s and 30s, thus born in the 1930s/40s, had a robust distinction between long and short vowels. However, the set that was pronounced with long vowels in initial position was a bit smaller than the (newly-formed!) standard - long vowels were already becoming short vowels.
- The proportion of native Seoul speakers that retain the contrast reduces. Kang (2015) gives a good overview of many studies from the 1980s to 2010s; fewer younger speakers retain the length contrast, fewer female speakers than male speakers retain the contrast, and of those that do, the length contrast is operative for fewer words.
- There is also some hypercorrection going on with older speakers, according to Kim, S. (2003) in the 표준 발음 실태 조사 2, as reported by Kang (2015).
- Kang (2015) finds that:
the change is almost complete, such that the long/short ratio falls below 1:1 and plateaus out in the youngest speakers’ speech, suggesting that we are observing the end stage of an S-curve in this change.
- The study also reports that distinctions even between high-frequency words are being lost as fast or even faster than low frequency words.
Will the Seoul Korean of 2021 join Jeju-eo and the Hwanghae region in North Korea as having neither contrastive tone nor length distinctions? It is certainly a speech variety in great flux through the 2010s, with some studies suggesting that it may in the future develop lexical tone. Cho (2018) reports that there is already a contrast: the high-toned 일.
Distinguishing By Length: Jeolla-do / Jeonnam 전라도/전남 사투리
According to the map, more rural Central-region dialects, Pyeong'an dialects, and Jeolla dialects are the other ones that (traditionally) have phonemic vowel length. Gyeongsang and Hamgyeong dialects generally also distinguish 밤〮 (night) vs 밤〯 (chestnut) by tone rather than length, much like Middle Korean is recorded as doing; having said that, at least in some Gyeongsang varieties, a difference in vowel length accompanies certain tones (rising tone specifically means longer length both in Daegu and Busan).
As for Southern Jeolla (Jeonnam) dialects as reported by Ko (2001), the sample of three used born in the 1970s could all use "distinguish the short from the long vowel both in terms of perception and production". According to Ha (2020), it seems that Jeonnam teenagers born around 2000 have mostly lost the distinction too though.
There are studies that say that Southern Jeolla dialects (focusing on Gwangju) can be analysed as a system based on phonemic stress rather than vowel length. This position on various changes that happen to the length of words, as well as the interaction of "intensity" with length and pitch.