Do I warn the players or not?

That's probably for the best. The number it produced at lvl20 was just a hindrance and a map-filler. Nothing wrong with the strategy used though.

For future reference, you really don't have to tell a lvl 20 party anything as part of a warning. 

There is a reason nobody plays L15+ without a DM comfortable with homebrew and adjudicating rule interactions on the fly—WOTC has pretty much admitted they stopped balancing around Tier 3 (not in so many words).

The rules @ L20 are broken. If I was running a one-shot @ L20 (I often do), starting stats 17/16/15/14/12/12 (commonly referred to as Epic Array), 1 Artifact / 2 Purple / 2 Blue, and 5k gold for bags of holding and brooms is my typical instruction to players. They are usually really fun sessions.

Then again, my player pool normally plays well past L20 (2CGaming Rules).

If you are setting them up to fail on a one-shot, why run it? The ethos of D&D is everyone should have fun. Tell them the theme of the encounter and let them prep for it—then it's a fun battle of wits between you and the group.

Just my $.02

Edit and btw if they are a group that has not played together before and nobody rolls a healer - I have a healing cantrip already in D&D Beyond its a BA heal 4d10+10 range 120' available to all casters - otherwise at those levels without a healer that knows what they are on about of course the group will die. 

Just gonna say even though I haven't read any of the replays to this topic and as a DM the best answer is NO.

I would tell them the basic premise before you go.  One paragraph.  And I would maybe give them five options of what it might entail and to be ready for all five.

And then go with Option #6.

☕️

Yes