There are increasingly few situations in which length extension really matters, because we know more about designing protocols than we did 20 years ago; even code that uses SHA3 or Blake2 tends to still use HMAC. Further, there are standard variants of SHA2 that don't have length extension (they're truncated and don't disclose their full state). It's better to not have length extension properties than to have them, but it's not really a big part of the desiderata.