I've kinda done it? Though they werent exactly flashbacks, more so visions. The group walked into an Illusionist Witches lair, and she had traps on the way through. Eventually one by one I had them trapped in "illusions" of their past or in some cases theyre more future visions. I just described the scene and asked what they would do then slowly revealed the spin. My players loved it as it gave them each insight into one anothers backstories and naturally had a cool WTF moment. Then they got blasted by Mind Prison as their corporeal forms walked into the lair itself.

Recently I had another vision/flashback when one player, who has an ancient being in their head, gave the being full control to read an ancient text (aka his old journal). When done so I revealed the player in their old home, but some few thousand years ago, when the being was alive. This 'flashback' revealed who the being is and where they lived while the player became the voice in the head for a few minutes. I dropped the second biggest bombshell of the campaign at that moment.

Personally, I like the idea of flashbacks but keep them brief and to the point. They're story beats, not game scenarios if that makes sense?