Not really, to be honest. Haskell gets hype well out of proportion to its real impact.

What are the key ideas that make Haskell unique? I'd say they are pure functional by default, lazyness by default, an extremely terse syntax that relies heavily on symbols, effects in the type system, type classes, and a few other smaller things. As a non-Haskell expert at least, these are the ideas I associate with the language.

Writing code in a more functional style is steadily becoming more popular, but functional by default does not seem to be the choice of any modern language targeted at the mainstream. Swift, Rust, Kotlin, Ceylon, Go are all examples of industrial languages developed outside of academia that do not follow this design choice, and their functional programming support primarily consists of map/filter/fold/etc type methods on collections and sometimes (as with Kotlin/Java) a lazy sequences library. Meanwhilst, whilst often convenient and intuitively helpful, the evidence that fully pure functional code is of much higher quality than mixed or mostly imperative code is ... lacking. Certainly it's difficult to get clear evidence without controlling for other factors like experience of the programmers.

Lazyness by default is an equally un-influential choice. It seems Idris is the newest cutting edge academic pure FP language and it's strict by default. Lazyness by default creates an entirely new class of bugs that developers didn't have to think about before, for little obvious benefit (it lets you write in a certain style more easily, but does that style really give better software? again, it's difficult to get robust data on this).

The extremely terse syntax that relies heavily on symbolic operators is widely regarded as a bad choice. Modern industrial languages like the ones I named have all shunned this type of design and Kotlin, for instance, doesn't let you define arbitrary new operators. Haskell code is often rendered much harder to read than necessary due to its abuse of symbolic operators.

Effects in the type system do feel nice, but the impact of this idea has likewise been limited. You could argue that Rust's borrow checker is perhaps somehow inspired by this, although it's sufficiently different that I'd describe it as original work rather than idea that comes from Haskell. Other languages haven't focused on it so much.

I suspect the next idea to cross over from Haskell will be type classes, or a variant of them. But probably as an obscure feature not many people use, rather than a fundamental building block as in Haskell.

All in all, given the level of attention Haskell has received for many years, I feel its ideas have had surprisingly little impact.

With respect to "serious production usage", that was obviously a lax description. As far as I know there's much more usage of Scala in the non-academic world than O'Caml or Haskell, but it's still quite a small amount relative to languages like Python or Java. I don't have any specific thresholds, it was an intuitive description.

> In an existing language that uses null pervasively, it would be hard to get a critical mass of legacy code which uses optional.

Exactly, but that's what Kotlin is trying to do through its auto-convert from Java to Kotlin, the backwards compatible type system, support for nullity annotations and so on. It's got the closest yet.