Star Trek: If Q Is Supposed To Be Immortal, Why Does He Die In Picard?

gamerant.com:

One of the greatest gifts the Star Trek franchise has given audiences over the years, other than some of the best examples of sci-fi technology beyond its years, is its incredible array of characters. There are, of course, some who never truly sat right with audiences, such as overenthusiastic chef Neelix in Voyager, or know-it-all whiz kid Wesley Crusher in The Next Generation. However, there are many more characters whom even casual watchers fell in love with. Picard, Sisko, Quark are some noteworthy ones, but there was of course the near-omnipotent, immortal Q. As it turned out though, this godlike figure is not so immortal after all.

While the Picard TV show of recent years has been met with mixed reviews, it has done a lot of carry on the stories started during the days of The Next Generation. Life after Starfleet at Château Picard explores the attempted retirement of the famous captain himself, but season 2 decided to show the stories end for one of the franchise's greatest antagonists-turned-friends: John de Lancie’s Q. The Q have always been suggested to be god-like beings of immense power, lording it over the majority of the universe. They have been around for forever, and are suggested to be impossible to kill — except by another Q, a concept explored in a Voyager episode. They neither age nor die, but in Picard, something interesting happened. The series focused around the idea of Q deteriorating and using what was left of his power to do what he does best: mess with Picard while trying to teach him an important lesson.

Star Trek: What Would Happen If The Borg Could Assimilate A Q?

There is, with many things Star Trek, an in-universe explanation created to make logical excuses for an out-of-universe