Now that I think of it, are there any major startups launched post Rails glory days? Some time around and before Rails 4.0, so 2013.
Almost all the major former startups that I could find are from 2010 or earlier.
Depends on your definition of major.
For a company to become a unicorn it usually takes 7 years (not sure on the number but it keeps changing). So many companies formed in 2015,16...+ aren't there still. Looking in Linkedin for Rails jobs there are a lot, not all of them are old companies.
Gitlab is a very famous name(2014) but it too IPO'ed so it's not a startup anymore.