SpellObjectEnthusiast
Adventurer
So, to other DMs, if in the middle of combat players decide they should retreat, how would you run it?
Part of the social contract of 5e is that the PCs will not flee from combat, and that the GM is designing all combats to be winnable.
Even "Deadly" encounters should be easily within the capabilities of most parties.
5e is an attrition game. The choice of whether or not to flee is not made during combat - it is made between combats when the party is considering their overall daily resources (spell slots, hit dice, etc).
Character deaths and TPKs should result from the party's unwillingness to retreat+recover, not due to GMs setting encounter difficulties above recommended thresholds. Fleeing during combat is purposefully difficult - once you've entered combat, you must finish it, for good or ill. Attempting to flee will lead to death, always.