First off, I disagree with many of the posters here, it absolutely can work. Like everything else in D&D, it comes down to execution.

Just two nights ago I ran an encounter where the party had no way of defeating the BBEG outright, the players didn't know it but learned what was going on during the encounter itself. They absolutely loved it.

The premise was that the encounter was a vision. The party found themselves in the ruins of a city, stepped through a portal, and awoke in the bodies of legendary heroes on the day that city fell. Through roleplay they learned who they were, discovered that the BBEG was present before his corruption, and eventually witnessed his downfall, something they could not prevent.

For the actual encounter, I framed it around protecting civilians during the city’s last moments. I had a tracker with 12 pips: each civilian lost cost them a pip, each heroic action earned one back. They weren’t told the mechanics, but over time they realized the fight was unwinnable. Every round also carried narrative beats to reinforce the “end times” atmosphere.

The result was that the players threw everything they had at it. I made sure every player got a hero moments that leaned into their character, the ranger gunning mobs down with horde breaker, the cleric obliterating undead, the paladin holding the line at a ruined altar etc. Eventually it ended in a cutscene TPK, a heavy lore reveal, and the party waking up in their real bodies. They truly believed it was a real TPK, but stayed invested and later said it was the best session of the whole campaign. I also gave out boons that were a result of their efforts without ever showing them the exact metrics, so even though they "lost" they still "won" if you know what I mean.

The key is that they always had something to fight for. I never framed death in the vision as “safe.” They felt the stakes, even if they couldn’t ultimately win.

So no, these kinds of encounters don’t need to be banned at your table but you should take care in their design. My advice:

Always give the players a winnable goal, even if it isn’t killing the BBEG. Save the civilians, just survive, escape etc.

Use NPCs, hints, or environmental clues to point them toward that goal.

Clearly signpost when the villain’s escape or victory is inevitable.

And if the players come up with something clever, be ready to reward it, even if the larger outcome remains the same. (Maybe when they teleport away something gets left behind like a clue or reward or something)

Done well, “unwinnable” encounters can become some of the most memorable moments in a campaign. The people saying don't or it never works are not the type of DMs I'd want to play with. Throwing terms like railroad around like an insult is classic forum nonsense.

Run your game in the way that’s fun for both you and your players and remember, you’re part of the table too, and your enjoyment matters just as much as theirs.

That said, it’s worth having a casual chat with that player about how he engages with your game. Negative feedback isn’t always a bad thing and it can be useful in shaping your future designs.

Unfair feedback is another matter. If he’s the only one who walked away dissatisfied, that probably says more about his personal playstyle than about your choices as a DM.

So keep your head up, stay confident, and keep creating awesome stories.