But both Mojave and Catalina had their last update in July.
The question wasn't whether 12.7.6 will be the last update for Sonoma, but when the support will end.
According to Wikipedia, Mojave was officially unsupported as of October 2021 (the same month Monterey was released), and Catalina was officially unsupported as of Nov. 30, 2022* (one month after Ventura was released on Oct 24, 2022), indicating if there were a significant and patchable security threat after July of those years, an additional update would have been made.
Further, the actual last security update to Mojave was on Sept 13, 2021 (Safari 14.1.2) and to Catalina on Aug 18, 2022 (Safari 15.6.1). So while these were not updates to the OS, these were security updates to a core OS-associated app, indicating continued support.
*Given this, I should edit my original statement from this:
"Based on past practice, Apple will continue to provide security support for Monterey until Sequoia comes out..."
to this:
"Based on past practice, Apple will continue to provide security support for Monterey
at least until Sequoia comes out,
and possibly for a month later...."