At the end of Avengers: Endgame,
Tony Stark dies, and his Arc Reactor shuts off.
I understand that it is emotional from a narrative viewpoint, but what is the in-universe reason the Reactor did that?
The arc reactor and the suit has just undergone a massive energy surge from the power of the Infinity Stones, even shielding Tony from them somewhat. It is highly likely that it was damaged beyond repair because of this and so also "died" at a similar time to Tony.
Alternatively it could be possible that as the suit is monitoring Tony's health, we see F.R.I.D.A.Y. report to Pepper about it ("Life functions critical."), it is possible that Tony installed a dead man's switch in the suit. I.e. if it ever detected the wearer had died inside of it, it shut down so that bad guys couldn't use it. I don't really buy this theory though and the former is far more likely.
The out of universe reason is obvious of course. It turning off is to symbolise and to show the audience that yes, he is indeed dead. Not just Tony that has died but Iron Man as well.