Does anyone know if it's possible to find out on what date a given microbadge was added to the list of available microbadges available for purchase?
The closest you can get is to click the microbadge, sort the list of owners by 'Recent', go to the last page and check the oldest date of acquisition. The microbadge is at least that old. Another 'method' on garnering 'confirmation'-(and closely related), is where you peer within the 'purchase(s)' and the DATE-shown for that, regarding such. For those from when this initially began, then it shall appear as a 15- transaction, with more recent ones being at 8- apiece now.
I'm also wondering if there's a way to list all microbadges created since a specific date.
Don't think so.