I have a friend who's been a AAAA veteran catcher with experience (now retired). From talking to him, I really think it varies. My sense is that as professional courtesy, teams are pretty open with their opt-out candidates on what they are thinking.
He also has said that while teams aren't "allowed" to reach out to someone who has an opt-out with another team, it happens pretty regularly. That Keuchel is still in St. Paul suggests that didn't happen, at least with a team he wants to go to.
FlyingFinn notes that they agreed to push the date back, which also often happens. We don't know how that conversation went. It could have easily been with a commitment that he will be the next guy called up if there is an injury (and it can be worked out with when he's last thrown in St. Paul, etc.). It could also be with an understanding that the Twins would willingly let him go if a team reaches out. It could be that they've told Keuchel they're seeking to trade a pitcher for a bat. It could be with....
And it's counterproductive for the Twins to screw him over or not be straightforward in a situation like this. If they do or aren't, word gets around and they won't have an opportunity with the next guy they are interested in.
I also don't think it's a coincidence this is happening with a Boras client. An agent is working for the player, obviously, but that doesn't mean it has be adversarial relationship with a team. I get the feeling that the Twins have a good working relationship with Boras.