Hi
I believe vlan-id none is used for the vlan translation in the newer 17.3 and later for vlan aware services. You can get more information in the below link
https://www.juniper.net/documentation/en_US/junos/topics/example/evpn-based-services.html
Starting with Junos OS Release 17.3R1, VLAN-based service with VID translation as described in RFC 7432 is supported. This means that Junos supports VID translation and the customer can have a different VID for each VLAN. As described in the RFC, the VID translation must be performed at the egress PE device while the MPLS encapsulated frames should also retain the originating VID. Figure 2 illustrates a topology where CE devices use different CE-VIDs for single VLAN-based EVI.
For more information on configuring VLAN-based service, see Configuring EVPN with VLAN-Based Service.
The following is a sample configuration for a single VLAN-based EVI. The same VID is used on all the PE devices, so VLAN translation is not required. In this example, the VLAN-id=none statement is included to remove the originating VID and to set the Ethernet tag ID to zero in the MPLS frame.
Hope this helps