I think Aucald's and Doffen's responses explain it well enough, but beyond that
There's no proof of this if you are saying the Silver Covenant left the Alliance at the start of Legion. Considering that in BfA we also have their faction as opening portals to old Dalaran in the new "as of BFA" summoning rooms.
I think continuing to latch onto the idea that "the Silver Covenant are a neutral faction" just shows the lengths of ignorance one is willing to go to reduce the presence of High Elves on the Alliance.
The Silver Covenant is
1) Only friendly to the Alliance
2) Reputation that can only be earned by Alliance members
3) Kicks out Horde players from their section of Dalaran for not being an Alliance member
4) Has its members providing aid to the Alliance throughout numerous occasions and has shown delight in killing Horde members
I'm not sure how one can continue to say they are neutral besides not wanting to admit that they're wrong. It's such an obvious thing and I don't take you for missing the obvious. But you have shown yourself to resist admitting when you're wrong.
It would be like someone trying to say Hozen and Jinyu are neutral factions to Alliance and Horde.