This was resolved by ef1ba8d, and released in 0.20.0-mc1.18.1

Probably do this by allowing a custom permission node to be created for each warp. E.g. essentialcommands.warps.<warp_name>

Hmm. But this has the downside of requiring players need permissions for each warp, which is annoying.
Potential solution:
Normally, warps are open to everyone. You have the option when creating a warp, however to make it a "restricted" warp. If this option is used, the permission is auto-generated based on the warp name and is required to teleport to it.