Phil Coulson (Clark Gregg) is still alive in the Marvel Cinematic Universe, and Captain America: The Winter Soldier confirmed that Steve Rogers (Chris Evans) has a high enough clearance level to know this fact. The two barely shared any screen time in the MCU, but Coulson's admiration of the American hero was firmly established in The Avengers. Coulson collected vintage Captain America trading cards and wanted Steve to sign them, but he never got the chance to do so.
After being a key piece of connectivity through Phase 1, Agent Coulson died in The Avengers at the hands of Loki. The heartbreaking loss was felt by fans and MCU characters alike, with Nick Fury (Samuel L. Jackson) using it to motivate the heroes to suit up and work together to defeat Loki. However, Coulson's death did not last long, as Marvel TV revived him so he could star in Agents of S.H.I.E.L.D. The series re-established Coulson and S.H.I.E.L.D. in the Marvel Cinematic Universe, but the movies have yet to really acknowledge it. This has frustrated plenty of fans who want to see Coulson reunited with the original team - or at least show that they know he is still alive.
Well, in Captain America: The Winter Soldier, a small detail confirms that Captain America should know that Coulson is around. S.H.I.E.L.D. compartmentalizes information on a need to know basis through their Clearance Level numerical system, that goes from 1 to 10. At the Triskelion (the S.H.I.E.L.D. headquarters in Washington D.C.), all of their elevators scan whoever steps on to make sure they have the clearance to go wherever they request. When Fury decides to show Steve what Project Insight is, they have to get on an elevator to go down to the hangers the helicarriers are being built in. The elevator confirms that Fury is a Level 10 - but also that Steve is a Level 8.
Level 8 clearance is a big deal within S.H.I.E.L.D., as there are only a handful of characters known to be above a Level 7. So, what does Steve being Level 8 have to do with Coulson's secret revival? It is established in the pilot episode of Agents of S.H.I.E.L.D. that knowledge of Coulson being alive is information that is available to Level 7 agents, such as Grant Ward (Brett Dalton) when he gets the promotion. With the necessary clearance level and his overall importance to S.H.I.E.L.D., Captain America should know that Coulson is alive and active.
Even though Cap does technically have access to this information, that doesn't mean he actually does know. Ward was clued in to Coulson being alive because he was being recruited to his team, but it is highly unlikely that every agent who moves up to Level 7 receives a briefing on all the information, like Coulson's resurrection, they can now know. Instead, it is probably up to the individual to seek out this information, which could mean that the only reason Cap doesn't know that Coulson is alive is because he isn't searching for such an answer.
The only other potential caveat here is the additional safety protocols - known as Level Alpha, Omega, and Tango - that can categorize information on another need to know basis. Only Fury and Alexander Pierce (Robert Redford) had Tango clearance, and it is possible that Coulson's revival was further hidden through one of these clearance levels.
That all said, does this mean that Captain America not only should know that Coulson is alive, but that he actually does? Gregg already believes that some of the Avengers do know that he is alive, so it is possible that Cap does know. If Cap does know, would any of the other Avengers be clued in? Black Widow (Scarlett Johansson) is also a Level 8 and known for keeping secrets, so she could have known this entire time. Considering how affected the team was by Coulson's death though, it is difficult to imagine that one member would not tell the rest about the good news at some point during their adventures. Since Marvel TV head Jeph Loeb has teased a surprising resolution to this mystery, maybe Avengers: Endgame will confirm who does and doesn't know. And if not, then fans can always believe that this has already happened off screen to give them a happy reunion.