How to get your AddOn removed
From ESOUI Wiki
In the past there have been some black sheep that did something which ZOS has explicitly taken a stance against for one reason or another. This page will list what your addon is not supposed to do, or otherwise it may get removed and make life harder for other authors in case some API gets locked down. If after reading this you are in doubt if something is okay to do, you should just ask on the esoui forum or gitter chat before spending time on something that may get removed later on.
What to expect
In case you do something that ZOS doesn't want or that goes against their ToS, you can expect various actions that will ensure the unwanted behaviour can no longer happen. They can range all the way from a simple request to stop doing that, to changes in the addon API that will have a negative effect on anyone who used it for legitimate purposes. In the most severe cases they may also hand out some bans to authors and users.
This one should be obvious, but in the past someone still thought it would be a good idea to silently mail gold to his account, which is why we now have a confirmation dialog for mails with attachments and one less player in the community.
ZOS does not want addons to communicate with any source outside the game in real time for a plethora of reasons. That's why there are no functions available to open a network connection or write directly to disk. They also don't offer any official way to do automated realtime communication ingame, but some of the hacks that use e.g. map pings in groups are tolerated for now.
Changing Gameplay Mechanics
They don't take kindly to any attempt to change how certain gameplay mechanics work. Like simplifying combat by automating parts of it or warning users about actions they should not be able to know about (e.g. attacks from stealth).
Using private or otherwise locked functionality
Anything that is marked as private is out of the question (although there are some exceptions where they simply misflagged something or forgot about unlocking it after making a change). Even if you find a way to still use it, you should be aware that this is considered an exploit and they will close whichever loophole you found.
Anything that affects server performance
Some APIs provide a lot of data directly from the server (e.g. guild history). If you plan on using them, you should think about the implications in case your addon gets popular and make sure you minimize the global performance impact in case thousands of users run it at the same time. Otherwise they may be forced to lock down a whole part of the API in undesirable ways (e.g. guild history).
Annoying other users
If the whole purpose is to annoy other users via some functionality provided in the game (e.g. rainbow colors in chat, spamming vote dialogs, etc), you can be certain that this will just lead to the addon getting removed and the api being limited so it is no longer possible to do that. There have also been some cases where someone found a way to crash another players client by sending them some forged chat messages or api requests, which will lead to you getting banned faster than you can say "It's just a joke, bro!".
Copying code from other AddOns
This is less about ZOS and more about other authors. If you copy something from another addOn, make sure to ask the author for permission first, otherwise they may issue a take down request with ESOUI, since their code is their intellectual property.