I'm interested as to whether it is more of a 21/20th century phenomenon.

That seems to be the case, but this question is better suited to /r/AskScience. Here are some previous posts that may be of interest

• Have intense allergies and/or hay fever always been as prevalent as they are now?

• How did people deal with allergies before modern medicine?

• How well did people understand the nature of food allergies before the modern period? Were deaths from anaphylaxis more common?

• Woke up with a massive sinus headache this morning. While taking my allergy meds, I got to thinking. What did ancient cultures do to handle common medical problems like allergies, heartburn etc? - a link to an /r/AskScience thread

• How did people deal with allergic reactions before epi pens? Did they just die? And how common was it?

misc cultural references

• Were allergies a problem for Romans and Ancient Greeks like they are for us nowadays? If so, how did they deal with the issue?

• How well were allergies understood by regular people, historically? - 9th c rose fever

• How much did ancient cultures know about food allergies? - King Richard III strawberries

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