One of the wildest "entire audience has the same reaction at the same time" moments I've ever had at the movies wasn't "On your left" or Cap with Mjolnir in Endgame.
It was the fate of Bing Bong in Inside Out as an entire audience of adults suddenly had the same realization, simultaneously.
I saw Brooklyn with an audience of mostly middle-aged couples, and when the heroine opened the drawer and we saw all the unopened letters from her husband, the entire theater went "Hmmm" like Marge Simpson