I learned you're supposed to introduce people by saying "She is Miss Wang," in Chinese, not "This is." Is it a regional thing?

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

This sentence reminded me of the mnemonic ”王伯伯、白小姐坐在石头上“ for the word 碧(bi4) - blue, bluish-green, jade or jade green colour.