Originally Posted by WoW Terms of Use
Additional License Limitations.
The license granted to you in Section 1 is subject to the limitations set forth in Sections 1 and 2 (collectively, the “License Limitations”). Any use of the Service or the Game Client in violation of the License Limitations will be regarded as an infringement of Blizzard’s copyrights in and to the Game. You agree that you will not, under any circumstances:
[...]
D. modify or cause to be modified any files that are a part of the Game Client or the Service in any way not expressly authorized by Blizzard;
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This is in the End User License Agreement, which you accept after each patch or installation (or find yourself unable to play); thus, this is something you should've already read and consented to.
Using addons doesn't modify game files. Changing models does modify game files. Therein lies the difference.
Also, the reason this rule exists is to prevent things like making the Peacebloom graphics 5000 times larger, essentially allowing you to reach and pick that herb from unfair distances compared to those without modified files. If you were to try that, you
will get permanently banned. For changing your character model aesthetically, without doing it in a manner that gives you an unfair advantage over others,
is very likely to get you banned, but occasionally you may just be suspended for a while. Regardless, it's not worth trying.