I feel there's a difference between "last time I flew a plane which is now obsolete, but was in regular use then", and "my last flight on this was when it was essentially obsolete" (both are interesting!). I haven't been flying long enough to have any of the former (although I'm up to my third obsolete airline now Flybe's done), but I can offer something in the spirit of the latter. October 2017, I made it onto the final KLM Fokker 70 flight, which I wrote up as a trip report. I guess the A318 is fairly niche, too, which I flew in BA's all-business config back in 2015.