Discovery also had hologram communication technology that I guess was also a secret? Starfleet went back to flatscreens for everything and didn’t use holograms again until the 24th century.

In "The Undiscovered Country" we see the Klingons are watching the Federation President's discussion with Azetbur using a grainy hologram. If they're able to receive a holographic signal, that implies that the Federation is transmitting one. Hell, even in the TOS episode, "Return of the Archons" when confronted with the holographic projection of Landru, Kirk and Spock recognize it for what it is right away, but the things they remark upon are the fact that there's no visible projectors, and Kirk says it's "Beautiful."

I don't think it's too much of a stretch based on what we've see to say that Starfleet decided that holographic projections were too low fidelity compared to viewscreens.

Hell, it even happens again. As you note, they made another attempt at holographic communication in the 24th century, which we see in DS9 the Defiant is kitted out with the new holo-communicator, allowing a fully realized, high fidelity, holodeck quality real time holographic communication. And where else have we seen it? We never see the Enterprise E use that technology; In "Nemesis" Shinzon is able to broadcast a hologram of himself from the Scimitar to Picard's ready room, but he claims it's through the use of his own holo-emitters. We've never seen it in LDecks, PRO, or PIC, all of which take place after DS9.

So yeah, Starfleet went back to flat screens for everything.