A limited subset of HTML is supported on Stack Exchange. For details on what is and is not supported, see What HTML tags are allowed on Stack Exchange sites?. The italics tag <i> is indeed supported.
However: the primary formatting mechanism on this site is Markdown. Using HTML is not discouraged, but seriously... why would you use it? This SEDE query shows up 23 posts of yours with <i>, and for none of them can I see a reason to use explicit HTML <i>s instead of the recommended Markdown.
In any case, the Ask A Question page explicitly discourages mixing HTML with Markdown, so maybe the problem came from that? Presumably the post in question is this one, which does precisely this mixing. Moreover, the list notation you used there, 1), was not recognized as a list by Markdown until the recent change of renderer, so it went from not mixing HTML and Markdown to mixing them; that could explain the change in behaviour. On the other hand, the rendered revision I just linked to displays just fine on my device $-$ but then again, the Markdown parsers are currently a weird mix, so who knows exactly what happened there?
.... but ultimately: who cares. The post renders just fine if you remove the html shenanigans. Use the recommended Markdown and this type of issue doesn't have the opportunity to arise.
As for the possibility of there being a fraction of posts whose formatting changed due to this edge case (or some other similar one) of the Markdown renderer migration, I guess it is worth taking a look to see how many such posts there are. A broader query finds ~500 posts site-wide with <i> in the body (though this may be closer to 800?). If you want to have a dig, that is one place to start.