According to Looper,
Owing to various production difficulties brought on by pandemic-era restrictions and guidelines, Season 11 of “Blue Bloods” consisted of just 16 episodes, far less than the usual 22 a broadcast network series generally produces. Ratings had also fallen to their lowest point in the show’s run, so when showrunner Kevin Wade got word from CBS executives in November 2020 to prepare to write and produce a double-length season finale, he presumed the worst. “I immediately took [that] as they’re planning to wrap up the series,” Wade told Deadline. And so he prepped for the two-episode, back-to-back-airing season finale as if it were a series finale. The plot concerned an uncharacteristically high-stakes mission for the Reagan family, with everyone coming together to protect previously long-lost nephew Joe, an undercover ATF officer, so that his cover isn’t blown.
The episode left some plot lines open or unresolved which ultimately ended up being an amazing thing because CBS decided to renew “Blue Bloods” after production on Season 11 ended.
Let’s hope they’re not thinking about ending it at season 12.