The 8th-level spell Demiplane lets one create a demiplane; further castings can re-connect to the same demiplane or to a new one. Additionally,
if you know the nature and contents of a demiplane created by a casting of this spell by another creature, you can... connect to its demiplane instead. (PHB p. 231)
So, what's the nature of a demiplane? What does one need to know to locate another's demiplane? Is "nature" a game term? The DMG's description of demiplanes (p.68) doesn't clarify it at all. Perhaps earlier Manuals of the Planes offer food for thought?
I.e. what's to stop a high-level wizard from just spamming the planes with Demiplane-requests? "Aahhh, I want to connect to that pocket someone created to store lots of gold pieces. Yeah... that one." [points randomly] "Oh, well, try again after nap."