Let's list all the removed portals and then pair them with the new 8.1.5 routes!
Hellfire Peninsula (Removed all of these for both factions)
New Dalaran (Legion) ALLIANCE portals removed:
Ironforge
Starting at entrance to make tower in Stormwind and ending at Ironforge Bank. There are many possible solutions:
1. Take twilight Highlands portal (at the Earthshrine where the Cataclysm portals are) to Highbank, fly MANUALLY to Farstrider Lodge in Loch Modan, then take flight path to IF: 5 minutes 40 seconds
2. Take Vashj'ir Portal to Silver Tide Hollow, take seahorse path to Sandy Beach, fly to Ironforge (its a straight shot): 5 minutes 30 seconds
3. Flying from Stormwind flight path: 4 minutes 20 seconds
4. Deeprun Tram: 2 minutes 20 seconds (lucky that I caught tram just before it left station)
***4 is the best option. TAKE THE TRAM!***
Darnassus
1. Take The Exodar portal in the new 8.1.5 portal room, take flight path to Lor'Danel in Darkshore, talk to Zidormi to go back in time and then fly to Darnassus.
2. Possibly the portal to Darnassus on the southern-most Stormwind dock by Perrin Moontear. (unless the portal is removed) (thanks to RobertMugabe for this add)
The Exodar
1. Take the Exodar portal in the new 8.1.5 portal room.
Vale of Eternal Blossoms
1. Take the Jade Forest portal in the new 8.1.5 portal room & fly to the Vale.
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New Dalaran (Legion) HORDE portals removed:
Thunder Bluff
1. Fly from Orgimmar.
Undercity
1. Unknown if there is still a portal to Undercity at the Orgrimmar Zeppelin, but possibly take the Silvermoon Portal in the new 8.1.5 portal room, fly to Brill, and then talk to Zidormi to go back in time and fly to Undercity.
2. Possibly take portal to Silvermoon and use Orb of Translocation in the room behind Lor'themar, you end up in Undercity, already in past phase (no need to deal with Zidormi), unless the Orb is removed. (thanks to Shauren for this add)
Silvermoon
1. Take the Silvermoon portal in the new 8.1.5 portal room.
Vale of Eternal Blossoms
1. Take the Jade Forest portal in the new 8.1.5 portal room & fly to the Vale.
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New Dalaran (Legion) Chamber of the Guardian common portal area - ALL portals removed:
Caverns of Time
1. Take the Uldum portal (at the Earthshrine where the Cataclysm portals are) and fly to Caverns of Time.
Shattrath
1. Take the Shattrath portal in the new 8.1.5 portal room.
Wyrmrest Temple
1. Take the Dalaran (Northrend) portal in the new 8.1.5 portal room. Fly to Wyrmrest Temple.
Dalaran Crater (Alliance)
1. Take the Deeprun Tram to Ironforge and fly to Aerie Peak, Hinterlands. Then MANUALLY fly to Dalaran Crater. (Flying from the Stormwind flight path will add 3 minutes)
Dalaran Crater (Horde)
1. Unknown if there is STILL a portal to Undercity at the Orgrimmar Zeppelin. Take the Silvermoon Portal in the new 8.1.5 portal room, fly to Tarren Mill, MANUALLY fly to Dalaran Crater.
2. Possibly take portal to Silvermoon and use Orb of Translocation in the room behind Lor'themar, you end up in Undercity, already in past phase (no need to deal with Zidormi), and then fly MANUALLY to Dalaran Crater, unless the Orb is removed. (thanks to Shauren for this add)
Karazhan (Alliance)
1. Fly from the Stormwind flight path.
Karazhan (Horde)
1. Talk to the Thrallmar Mage in the new 8.1.5 portal room and get sent to the Blasted Lands. Fly to Karazhan from there.
NOTES:
a. There is a ring that can teleport you to Karazhan. https://ptr.wowhead.com/item=142469/...he-grand-magus
b. Engineers have toys that can teleport them to Pandaria, Northrend, Argus, Outland, Everlook in Winterspring and Gadgetzan in Tanaris.