As said here, a modding no-no is something you cannot do. The list has changed a little bit lot since many years ago, with the development of workarounds and, most importantly, the Unhardcode Patch (UHC) and UHC Plugin (UPL).
The current list applies to the copies that are patched with UHC. If you don't use UHC then both the old list and this list apply.
* You cannot make new animations.
* You cannot train buildings. You have to build. Therefore Egyptian caravans are not buildings.
* You cannot transform units into buildings. They'll appear as foundation.
* You cannot spawn buildings. They'll appear as foundation.
* You cannot ship buildings. They'll appear as foundation.
* You cannot replicate the TownCenter, TradingPost and ypConsulate
* The tech effect "SetCivilization" disables access to the homecity. Revolutions are not tested yet. This effect has a relatively high risk to cause a crash.
* Negative trickles are impossible.
* You cannot make civ-specific musics.
* You cannot make situational sounds - like the revolution one. However you can trigger a sound using a tech.
* You cannot make models fatten. The three stages of a fattening sheep is actually three different models.
* Units cannot gather shipments and skillpoints.
* Units and buildings can't have a skillpoints cost.
* Truck attack always cuts trees down, even with 0 multiplier
* Knocked out units still trickle resources.
* Units stop trickling resources while in garrison. /!\ Didn't make enough testings.
* With hitpoints below zero, "KnockedOutDeath" units crash the game.
* Units will always be able to shoot through objects (walls, buildings, etc)
* Seems like units cannot have gather points. /!\ It sometimes work...
* You cannot determine game difficulty in random map scripts.
* You cannot query queued units in any script.
* Non-idle units can't be healed, unless you use autoheal without timer (like minutemen).
* You cannot put water in groupings.
* You have to put both ValidIdleVillager and AbstractVillager to make the "idle units" banner appear.
* Only buildings with placementrules\tradingpost.xml and/or tradingpost.tactics can receive resources from railroad units on trade routes. Replicating the files won't replicate the effects.
* You cannot decide what treasure will appear when place the treasure object.
* You cannot make natives nor Asians revolt.
* You cannot make units attack while moving.
* You cannot change a civ's flag without revolution.
* September 30th, 2018:
- Initial post
* September 30th, 2018 (several hours after the first post):
- Updated: "SetCivilization" effect.
- Added the old list's elements that are still no-nos.