If the purpose is to inform a human reader that a text involves AI, then in my view a tag is not the solution. People do not read tags as part of the actual content. The indication therefore has to be part of the context itself.
I personally use the method provided by Discourse:
<details class='ai-quote' open>
<summary>
<span>Title</span>
<span title='Discussion with AI'>AI</span>
</summary>
More or less content…
</details>
Alternatively, I use Quote Callouts component.
Teaching users to apply these correctly is, if not impossible, then at least extremely difficult for users coming from a Facebook background. So I have settled on requiring that AI-generated content must in some way be marked. As long as there is a visible indicator within the content itself and the AI material is clearly separated, I am satisfied.
I did try using a tag, but I noticed very quickly that it drew no reaction. In addition, I want to use tags for content classification, and it is hard for me to imagine that anyone would ever try to find all the content on my forum that involved AI. Especially since the use of AI is likely to escalate over time. It might even reach the point where a tag would be needed to indicate that content is human-made (and not just with an emoji, since such websites and even forums already exist).