It's definitely not dead. My best guess is that it simply lost the popularity contest to Medium. Svbtle's influence cannot be understated, however, in my opinion. I've seen a number of independent bloggers using their own "kudos circle" (inspired by the Svbtle feature)
I see why you think it might be dead, given that it stays quiet in the background and gets out of your way (as opposed to Medium for example), but that's the whole point of Svbtle.
Svbtle puts the content first - the content is the end game, where as Medium puts itself first; the Medium brand itself is put first; the content is just an afterthought.
I think OP was more pointing out that, at least from the outside, the product and its features appear unchanged from several years ago.
I tried Svbtle myself, and while I like the product aesthetically and conceptually, customization features that I really wanted like adding custom tracker tags just weren't there yet. I chose to keep using Ghost for now.
I would say Svbtle had gone experimenting. At some point I decided I wanted to be a part of something livelier and more inclusive and announced in a review of Book Worm Lab http://ratedbystudents.com/services/bookwormlab Svbtle dead too. But now I'm using it again:)
Working on it right now, in fact.