I am very much a don't give the dc let them know if they pass or fail it makes for a bit more tension and the inevitable table talk of them trying to figure out the DC which is fun to watch "ok so a 14 fails but your 17 passed earlier so likely a 15 but could be some random arsed number"

I don't tell my players the DC beforehand, but only because that would deny them the ability to use Luck Points after a roll is made.

Before, it makes things easier.

I don't believe players should know the DC, or even if they "failed" per se.

The player describes their action. (They don't say "I want to do an Arcana check.)

I ask for a check.

They roll.

I narrate the result.

I announce DC’s when it’s a crucial chance as an opportunity to add importance or stress to the roll.

Or halfway through combat once the DC has been established, like a Spell Save DC to help speed up combat

I asked my players which they prefer and they said they like not knowing what the DC is. They said they like the tension of not knowing how a scene will play out.

as a player my DM very rarely tells me the DC, but sometimes they'll ask if I have anything i can add to a roll or if I want to use my inspiration if it's something nasty

depends. in combat I won’t say the dc for the first couple, but after a bit I’ll say it to speed things up. especially for aoes I might say “anyone with 15+ saves.”

out of combat stuff I usually don’t. but sometimes I might say “beat a 10” or something

As a Player I insist I know the DC before I roll so I know the GM isn't fudging things. So, as a GM, I make sure Players know the DC before a roll.

I actually trust my players. I don't care what they roll. Please roll a 17 WIS save for X and let me know pass or fail. If you need to cheat to win DND, go for it.

I think it can depend on if they can see how hard it is before attempting for skill checks and usually not for saves. Like if they are planning an acrobat stunt check they have some idea how hard it would be, but a sudden stunt across a room of unknown variables might be hidden.

Same though for some activities if they make a successful Intelligence (Investigation) check I will give them the DC.

To name one example if there's a trap and instead of disabling it they investigate and succeed I'll explain exactly how the trap works and give them the DC (because the character would get a good idea of how difficult it would be to disarm it. I personally find that this rewards a careful approach and it gives them weigh the merits of avoiding/repurposing as opposed to disarming. It also gives characters without thieves' tools a way to deal with traps albeit a potentially less effective one.

Different tables may vary of course.

I mix it up. Sometimes yes, sometimes no.

I do for saving throws, so they know to use things like inspiration. I do not for skill checks. If something bad can happen, I want them to know the target. If they just miss out on something good, I don’t tell them— which fits my DM style of “sure, you can try” when they want to search a barren place or try to intimidate someone who won’t budge.

Generally no. I often have players with features that can be used after a roll but before they know the result so I don't want to render those invalid by default.