I'm a little bit of an insider of music industry in my country and I can explain the situation. I sing and I can say that T x t's vocal line as always been good, they never really had a sudden improvement. With good I definitely don't mean Jungk0ok level, which has an incredibly good control of his vocals, or Hwasa level or whatever lol
But they are good at conveying feelings in the voice, especially Ye0njun, except for T4ehyun who is more technical. I would say a medium level.
The problem with them is: they clearly lack training. Their stamina, especially Ye0njun's, is extremely low so you can watch a very good performance of them and also a very bad one the day after because they're exhausted.
For what live vocals are concerned, unfortunately they are totally playback. I've watched their live concert past week and it was all pre recorded.
This technique that we may use in the pop music industry (which is what SM does everytime with Sh1nee and Tae1m comebacks) is prerecording live vocals with stage mics while dancing or moving softly, edit them and then use the track for lipsyncing so that it sounds live. Of course this way it sounds very natural, but you still can tell because they're moving like crazy in some moves and perfect notes at those moments isn't really possible. Especially if you listen to a whole live and can never hear a mistake, which was T x t's case at the latest online concert, it is clearly pre recorded.
Flaws in vocals of a live performance are beautiful I think, it really bothers me this total lipsync, instead of at least putting background vocals with a high volume and singing on them, because they really are good.
As for what Taem1n is concerned, he had some months to train the vocals and SM has great vocal trainers, they all sing the same way but at least it works. He also said he practiced extremely hard to learn to the point of bleeding (I don't know if this statement can be trusted cause bleeding cant' lead you to learn singing lol)
What B1ghit care most about is the uniqueness of the vocals, so they don't spend money training idols because when you learn technique you could easily lose some of your voice quality for the sake of a correct singing technique. And they also don't think an idol needs to know how to sing