A friend who recently moved to Porto said he’s been using "prazer em conhecê-lo(a) when meeting people, but gets strange looks when he says it. He asked his teacher from Porto who said she would never use that, always "gosto em conhecê-lo(a) but she couldn’t explain why.
Is using “prazer” regional? Old-fashioned?
I’ve seen (and heard) “prazer” used in that way and others - like when saying, “it was a pleasure to work with you” or “it was a pleasure to serve you”. Certainly “gosto” works too. The phrase “prazer em conhecê-lo”, while I’ve heard it used and used it myself, isn’t really a complete sentence or thought though, imo: “pleasure in meeting you”, but rather a short-form version. Kind of like “my pleasure” in English.
I believe I’ve even heard it shortened down to the ultra-minimalistic, “muito prazer” and left at that! 
I can’t speak for Porto, but you’ll hear it around Lisbon, at the very least. I always heard it both ways my whole life and neither seemed better or worse. Like @pmfrancisco said, we do tend to shorten it to just “É um prazer”, “muito prazer” or “prazer” or “Muito gosto” (never just “gosto”), even in formal contexts.
Thanks for the information, guys! I’ll pass this along to my friend in Porto. 
You’re welcome