I haven't found any studies or papers that identify the regional distribution of the pronunciation of second with a phonemically voiceless final consonant by adult speakers. It seems to be a real phenomenon, but an understudied one. Here is a Reddit post from 2105 about it: "Are there any dialects where 'second' is pronounced like /ˈsɛk.(ə)nt/?" (posted by
u/backpfeifen-gesicht in the r/linguistics subreddit).
The paper "The Speech of Fourth Graders in Fifteen Central
Pennsylvania Schools: Phonological and Granmatical
Variables", by Maly Alice W. Minderhout
and David J. Minderhout, from June 1973, mentions secont, but only as a children's mispronunciation of second.
I also found a passage in an American periodical published Jan. 1, 1848 that uses seckunt to represent the pronunciation of the word in a bit of dialectal speech:
Presently, a man elbowed his way out of the pit, and accosting Burton said--"Look a here, I can't git a chance to see anythin', no how. D'ye think I could git inter the seckunt tier, sa-ay!
("Progress of Man Under the Republic", The John-Donkey p. 326)
Unfortunately, it's hard to tell exactly what seckunt here is supposed to represent:
- an alternative pronunciation that was common in the author's time and place, but considered nonstandard;
- a pronunciation that was common for all speakers, but only written down in "eye-dialect";
- an uncommon alternative pronunciation that the author just thought helped to produce the right overall effect;
- an allophonic or conditional devoicing of the final /d/ in second before the voiceless plosive /t/ at the start of the following word tier.
But I think this passage at least provides some evidence that pronouncing second with a final consonant that sounds like /t/ is not an extremely recent phenomenon in English.