> I guess this is why most wine carry a sulphate allergy label these days.

Partly. There is some natural level of sulfites present in wines - very dependent on the terroir, and then the rest comes during the manufacturing process; some sulfites may be added directly so the wine does not spoil.

There are wines that do not contain any added sulfites and even then may cause trouble to the sensitive persons if the sulfur content of soil is naturally high. Some manufacturers therefore also add info about total measured sulfite content to the label.

And nowadays, not only demijohns or wooden barrels are used to hold wine; of course stainless steel vessels, but also concrete vessels and even things like the Georgian qvevri underground clay vessels or amphorae are having a comeback.