https://screenrant.com/star-trek-discovery-mirror-universe-spock-death/

Star Trek: Discovery filled in some of the gaps in Mirror Universe history and revealed the fate of Spock after Star Trek: TOS" "Mirror, Mirror."

Why Ethan Peck Didn't Appear As Star Trek: Discovery's Mirror Universe Spock Revealed By Writer (https://screenrant.com/star-trek-discovery-no-mirror-spock-cameo-why/)

Star Trek: Discovery season 5 reintroduced the ISS Enterprise, and co-writer Carlos Cisco says there was talk of an Ethan Peck cameo as Mirror Spock.