I suppose it would make sense if I just list the products that I don't have an original release date for...
Brian, it's not too difficult to find dates for many of these. It does depend on how detail you want to get. One simple source (at least without going to archives) is the News section of sequential.com (going back to 2013). Another spot to check is introduction videos.
It you want launch dates, here are some from glancing at the headers of the News section (the Pro 3 was in my head, already, since I picked up an early Pro 3 SE) and some ship dates since they were also posted:
Pro 3 SE Jan 2020, shipped Mar 2020
Pro 3 Jan 2020, shipped Jan 2020
Prophet 12 LE Nov 2018
Prophet XL Oct 2018, shipped Nov 2018
Prophet X May 2018, shipped Jun 2018
Prophet Rev2 desktop Aug 2017, shipped Oct 2017
OB-6 desktop Jan 2016, shipped Sep 2016
OB-6 shipped Mar 2016
Prophet-6 desktop Oct 2015, shipped Nov 2015
Prophet-6 Jan 2015, shipped May 2015
DSM03 Feedback module shipped Oct 2016
Pro 2 shipped Aug 2014
Prophet 12 keyboard Jan 2013, shipped Jun 2013
Prophet 12 desktop Dec 2013
Prophet '08 desktop (let me guess, was it released in 2008?) Jan 2008, shipped Mar 2008
Mopho SE Oct 2013
DMS02 Character module shipped Jan 2015
DSM01 Curtis Filter module shipped Aug 2014
Cheers.