Older people living in some rural areas north of Venice use the voiceless dental fricative /θ/ for many words, like cena "supper" which is pronounced θena, exactly like in Spanish cena (Castilian, not Latin American) while in Italian the pronunciation is t͡ʃena. Other examples can be graθie for grazie, θereza for ciliegia "cherry" and so on.
The /θ/ sound was allegedly introduced by a Greek saint during his evangelization of those areas, and it remains as a legacy of his origin since it corresponds to the Greek letter theta. I've read than one can even trace the path followed by the saint by looking at the places where this sound is still being used, from Venice and up to Padua, Vicenza, Treviso, and Belluno in the Alps mountains. Towns located a few miles apart didn't adopt the theta at all.
I don't know whether this story is true or not, but I'd be happy if it could be confirmed! Anyway, despite its respectable origin, the /θ/ spoken in those parts of Italy is today unequivocally associated with poorly educated people, especially of older generations and living outside the big cities. The Venetian dialect and its variants are still widely used throughout the Venice region, including every urban area, but the vast majority of people never use the /θ/ sound, which has a decidedly "peasant" connotation. It is generally replaced with /s/ or /tz/, so in the example above θena becomes sena, which is perfectly acceptable in any conversation and in theatrical works spoken in dialect, where the use of /θ/ is absolutely out of question!
This stigmatization may be difficult to grasp for English or other non-Italian speakers, but it makes perfect sense for an Italian as it doesn't exist in our alphabet and it is so distinctly different from all the other phonemes we use.
My grandparents didn't speak with the /θ/, but sometimes they'd throw in words with this sound to stress the fact that they grew up pronouncing words this way but then they became more educated, and also to mock people that still speak in dialect this way!
Is there any other example of phoneme stigmatization, not necessarily associated with the level of education but in any other context?