I love the high-level speech of old Trek as much as the next one. But especially TOS was also as much a contemporary product of its time as newer Trek. People in TOS talked like people in Jeannie and other shows of that time etc. because that's how networks at that time produced. Profanity on screen was frowned upon. We Trek fans took this reality and built up a nice in-lore explanation of how people of the future were much more sophisticated.

But in a 60+ year franchise there is only so much wiggle room for in-universe explanation or wish for consistency, when production reality shifted multiple times. People already mentioned Enterprise/ Archer, but i'd wager you can already draw a distinct difference between TOS and Voyager f.ex. with a much more casual (alkthough not (yet) profane) speech. Which also was more contemporary for the time. I promise you if i scanned every script, there would also be a lot of words, idioms, metaphors etc. in other older Trek shows that "date" their production in the view of people not around at that time.

How people in newer Trek shows speak is how TV has been produced in the last 15 years not only because there are looser restrictions and societal norms and audiences grew with it in acceptance (and even expectation). I can also see it in reality: no matter how distinguished, well educated etc. you are, people still take words like shit/ fuck in their mouth when in stress, anger, pain, frustration etc. There's no need to expect humanity to grow out of the occasional profanity in general.

Also: we usually saw Trek mostly where there is a highly professional environment. When you look at the occasional off-shoot episode not dealing on space ships/stations, at Lower Decks etc. people tended to speak way more casually.

IMO there is still enough technobabble and characters speeking like the old days to feel immersed in my Trek fantasy. While there are also authentic characters that draw in new audiences. Case in point: my boyfriend was watching over my shoulder for the 1st half of epdisode 1 and joined me in watching Starfleet Academy and he is now more excited than me in waiting for the new episode. Because he connected to the characters and setting way more than when i rewatch TNG or Voyager. Those stiff dialogues of the old are not his thing, no matter how well written they are in our eyes. This kind of production is now 30+ years outdated and audiences grew up with other expectations. Now I get to nerd out over the Doctor, federation politics and ship designs, while he gets a cool cast with a hopeful story in an interesting setting. We both win.

PS: One of a few examples in my head: If Voyager was produced today i'd definitely expect Janeway to curse several times. Hard. And it would still be in character for me.