In popular terms "havelågebeskatning", formally the tax authorities actually refer to it as the "exitskat" (exit tax):

https://skat.dk/skat.aspx?oid=2176214

It is called “fraflytteraktieavanceskat”. It is a capital gains tax for equities that can be paid at once as if you had sold the shares, or you can delay it by filling out a form every year to the Danish tax authorities after moving abroad.

Capital gains tax is not the same as exit tax though?

Well capital gains tax is pretty much normal, it exists in most places.

As for exit tax in the U.S., i really wonder how one could enforce it. So if you left the country and have another passport (otherwise you couldn't drop U.S. citizenship), what can they possibly do to you? I don't think this absurd leftist idea even deserves much attention or comments, someone just said bs without thinking of it much.

You'd have to self impose exile.

You are required to enter the US on your US passport if you have one.

If you wanted to go back (say to the funeral of a friend, or something equally important) then they could detain you until you paid what they thought was due.

Well a person would not have a U.S. passport if they dropped citizenship right?

When you apply for an ESTA or a US visa you have to tell them about citizenships you hold or have held.