Nobody was banned for using Miat's spell alerts and the addon circumvented all stealth mechanics. I don't think you can anticipate ZOS behavior logically, as the owner of the game they have the right to ban/unban every programm they don't like, be it comprehensible (ToS, EULA, etc.) or not. So while I definetely think they should officially allow ENBs, I would hesitate using one as long as they haven't officially given any persmission to do so.
Don't get me wrong, I love graphical overhauls and I would love seeing it in ESO! So if you could get ZOS to officially permit it's usage or release a table of functions that are allowed or not allowed, it would be much appreciated, so I could use it myself