I don’t know their motivations, but having the same code work fine on one JS engine / browser / browser version, but totally stack overflow on another seems like suboptimal to me. Explicit tail calls let you error out properly at the call site, Babel-style downcompile it to a loop of some sort, all that stuff.

Generally the people that care about tail calls write code that relies on them. It isn't just an optimisation. The code is broken (will cause a stack overflow) if TCO isn't applied. So you need to guarantee it.

the same discussion is going on with rust. it lets the compiler enforce that you're actually using a tail call, instead of silently optimizing it when it sees it and otherwise being silent.

I quite like that Clojure’s recur makes it easy to tell at a glance if a recursion will be efficient.