While zsh, bash, mksh, ksh93, probably others have it, sure. But many don't -- and not totally irrelevant ones either. Debian's default, dash, for example, does not support `[[`.
IMO, unless you're writing something like shell-specific dotfiles, avoid non-POSIX features.
It's usually pretty trivial to avoid them, especially if you're willing to call other mandated commands like awk, etc. But often, with a bit of creative thinking, most non-standard features can be replicated with some combination of `set`, separate functions and/or subshells.
Shell scripts, in general, have dozens of footguns, are pretty much impossible to statically analyze, difficult to make truly robust, and many of the shells themselves -- e.g., bash -- have huge, borderline inauditable codebases.
I can think of a dozen reasons not to write shell scripts. Yet still, there is incredible value in the fact that some form of POSIX-compliant/compliant-enough shell can usually be found on most systems.
All of that value goes out the window, though, the moment you start relying on non-standard features.