Star Trek: Discovery season 2 retconned the classic Star Trek: The Original Series episode "Mirror, Mirror" because Trekkers now know Spock (Leonard Nimoy) was fully aware of the existence of the Mirror Universe. The twisted parallel dimension was the fourth episode of Star Trek: TOS season 2; it not only introduced the lasting image of the Mirror Spock sporting an unforgettable goatee but the Mirror Universe has been an enduring concept resurrected in memorable episodes of Star Trek: Deep Space Nine, Star Trek: Enterprise, and Star Trek: Discovery.
The second half of Star Trek: Discovery season 1 took Michael Burnham (Sonequa Martin-Green) and the U.S.S. Discovery to the Mirror Universe for several episodes. Even after they returned from the alternate reality, the Discovery had a permanent reminder aboard: Emperor Philippa Georgiou (Michelle Yeoh), the doppelgänger of Burnham's former Captain on the U.S.S. Shenzhou, joined them in the Prime Universe. Georgiou joined Section 31 and she played a major role in Star Trek: Discovery season 2's season-long arc to stop a rogue A.I. called Control from wiping out all organic life in the galaxy. Star Trek: Discovery season 2 also introduced the younger Spock (Ethan Peck), Michael's adoptive brother; the Vulcan remained behind when Burnham led the Discovery into the 32nd century and Spock wiped all record of his sister and her starship from Starfleet records.
But a Vulcan never forgets and Spock kept the secret of Michael and the Discovery for the remainder of his life — and that included his knowledge of Georgiou and the Mirror Universe. In fact, even Captain Christopher Pike (Anson Mount) hinted to Georgiou that he also knew about the parallel reality she originated from. All of this recontextualizes the events of TOS' "Mirror, Mirror" in regards to the actions Spock took aboard the Prime Universe's Starship Enterprise when Captain James T. Kirk (William Shatner), Lt. Uhura (Nichelle Nichols), Scotty (James Doohan), and Dr. Leonard McCoy (DeForrest Kelley) were switched with their evil Mirror counterparts thanks to a transporter accident.
Most of "Mirror, Mirror" focused on the Prime Kirk, Scotty, McCoy, and Uhura aboard the Mirror Universe's Enterprise and there was only one scene of Prime Spock dealing with his crewmates' Mirror doubles, but that crucial encounter becomes fascinating after Star Trek: Discovery season 2. The clean-shaven Vulcan quickly realized he was dealing with the Mirror Kirk and Spock wasted no time locking the evil versions of his Captain, ship's surgeon, communications officer, and chief engineer in the brig. Spock calmly informed Mirror Kirk that they would remain in custody until he discovered how to return them "to wherever it is you belong." Spock was also bemused when Mirror Kirk tried to bribe him with power and riches; the Vulcan found the stark differences between the Jim Kirk Spock knows and his doppelgänger fascinating.
Spock not only cooly handled the Mirror Kirk but the Vulcan remained tight-lipped about how he knew something was very wrong when the Prime version of Kirk and their friends switched universes once again. In the original context, fans simply had no trouble believing that Spock's brilliant mind simply deduced that "some kind of transposition has taken place" and he acted accordingly. But after Star Trek: Discovery season 2, Trekkers now know that Spock had prior knowledge of the Mirror Universe and he understood right away that the evil Kirk came from the same alternate reality that Emperor Georgiou came from.
With this retcon in place, it's interesting to see Spock's actions against Mirror Kirk and read between the lines of what he told the Mirror Captain. When the evil Kirk ranted and raved asking what was going on, Spock immediately realized that Kirk's double had no knowledge of the Prime Universe just as the Kirk Spock knows wasn't aware that there was a Mirror Universe. Spock chose his words carefully and he wisely gave the Mirror Kirk no information that could be used against the Prime Enterprise. The Vulcan merely said to Kirk, "I can answer none of your questions at this time." But Spock did know the answers!
Spock properly did what was necessary to protect the two alternate universes from each other in "Mirror, Mirror". Thanks to Star Trek: Discovery season 2, Trekkers now know Spock possesses much more information about the events of 2257 that he never disclosed and the Vulcan did his part to protect his Enterprise from encountering the Mirror Universe again.