In Spigot 1.13.2 the PlayerPortalEvent was called even when the nether is disabled. This is not the case in 1.14.2 anymore.

Relevant code in EntityPlayer#a(DimensionManager, TeleportCause):

WorldServer worldserver = this.server.getWorldServer(dimensionmanager1);
WorldServer worldserver1 = this.server.getWorldServer(dimensionmanager);
WorldData worlddata = this.world.getWorldData();
if (worldserver1 == null) {
    return null;
}

dimensionmanager is the dimension to which the player will be teleported. If the nether is disabled worldserver1 is null and the event (called later) is not called.

I am not 100% sure, but I think the event was also called when leaving the end in 1.13.2. This is also not the case in 1.14.2. The relevant code for this is above the code mentioned above.