The -20% bonus remains a huge mystery.
Woohoo!
EliteTroops really did
solve the mystery of -20% Trade Route bonus.
I fired up my old culture game that was driving me crazy, and sure enough my city revolt cause my island city to switch its trade route to one of the other 2 Legendary cities.
So it was NOT a bug at all when my Legendary City 1 came out of revolt and the -20% bonus vanished.
It just switch to a different Legendary City, because I only had 1 island city and my 15 pop city was taking 1 of its 2 trade routes.
Yes, if you can force one of your island cities to form a trade route with the foreign owned legendary city, you will get -20% Trade Routes bonus in that city!
This is the reason I never could figure it out before.
In past games, I'd always try to crush my
win civ down to 1 city or put it on an iceball island so it couldn't send a big army to occupy my legendary cities.
But this destroyed the +150% sustained peace bonus that my island city was getting when it tried to form a trade route to the Legendary City, reducing the trade route from 3
down to 2
.
That made all my mainland cities equivalent to my Legendary cities as far as trade routes go.
Also, if I had Open Borders agreements with other AIs, my island city frequently formed 3
per turn trade routes with any AI I had not been at war with recently.
That is why when I Closed Borders with all AI, I'd sometimes get -20% Trade Route bonus, but usually not.
Because my island city would just pick one of my numerous mainland cities to get +2
per turn with instead of the Legendary City which also gave +2
per turn because I'd been at war with that civ recently.
So if you want -20% Trade Route bonus in your spy city, force your island city to form a trade route to it!
1) Any civ that is getting +3
per turn trade routes with your island civ, close borders with them.
2) Build Great Lighthouse and tech Currency so the island city has more trade routes.
3) Try to have 75% Sustained peace bonus with your legendary city foreign civ so the legendary city will barely get a +3
per turn trade route to form to it from an island city.
4) If you have any 14 population or higher cities, whip them down to 13 pop >_>
A player with 4 high population cities will never get the -20% trade route bonus because all island cities will form trade routes to the 4 high pop domestic cities.
Sustained Peace bonus
recharges 3% per turn, so you need 25 turns of peace with the legendary gift civ before 3
per turn trade route will reactivate.
OR
You cheat, and build a +50% harbor in your island city.
This means you only need 25% Sustained peace bonus, so you can form the trade route with the legendary gift civ after only 9 turns of peace (+27%) instead of 25 turns of peace (+75%)
But if you do this, your island city will also form a trade route with any domestic mainland Size 11 cities or higher for 3
per turn.
Galleys and Combat III + March Berserkers because a strong coastal production city got 5 Settled Great Generals. 
5
every 2 turns, forever 
