"Mr. Li, this is Miss Wang."
I learned you're supposed to introduce people by saying "She is Miss Wang," in Chinese, not "This is." Is it a regional thing?
Nope. Both is acceptable, depending on whether the one being introduced is in front of you or not.
In China, 小姐 is less used to mean young lady because the meaning prostitute took over the original meaning; 女士 (nǚshì) is used instead. One can still use 小姐 as a title (see the usage example). Do not use 小姐 to call a waitress, but use the neutral word 服務員／服务员 (fúwùyuán) for that instead. The definition may revert somewhat in the distant future (becoming similar to the word 姑娘 (gūniang)). source: https://en.wiktionary.org/wiki/小姐