You're a little disingenuous with this. You actually said:
The term "boomers" is well known as referring to the baby boomer generation in a derogatory fashion. The content of your post supports this further.
Boomers is literally the short version of baby boomers. You may be thinking of when "OK boomer" is used as a dismissive response to an actual point being made.
I dunno. I think the "ok boomer" meme is perhaps what you're thinking of, being a catch-all term for outdated worked-out-ok-for-me thinking.
'boomer' itself isnt really a derogatory term. No more than 'millenial' as applied to all young people, rather than the correct age group (who are all aged at least 26)
This.