I get a few bulk-mail-stamped envelopes a week. Although stamped mail isn't as prevalent as it used to be, I wouldn't call it rare or scarce. If we start talking about non-machine-affixed non-philatelic commemoratives, then I think 'scarce' may apply. My (virtually) ONLY source of in-mail commemoratives is on stamp auction catalogs and on
ebay stamp purchases (and certainly not all of those are stamped). It is a calendar-circling-event when I get a personal stamped letter with a commemorative on it. E-mail, texts, and phone calls have replaced the letters of yore. Sorry, but mail simply isn't as interesting as it used to be. The great majority of it is bills and junk mail, with as many magazines as I dare subscribe to sprinkled in each month (no stamps, of course).