Does the perm "portals.portalname" still work to authorize individual portals?
Is there any documentation for this? I could not find anything current on this.

Version: 0.7.0

We've never used those permissions. If you want to set a portal to need a permission use permission:some.permission.here and it will check if the player has whatever is after permission:

We only use that if you enable the warp command.

If you want to check if a player does not have a permission you can use permission:!some.permission.here

Oh that's right. I had set the perm tag like this.

Is there any way to stop the teleport of the vanilla nether portals?

You would need to do it using multiverse or disable the nether. By default we disable portals in overlapping regions.

I use multiverse (and have disabled the nether) but there is a issue.

Multiverse/Multiverse-Core#2634