I'm not a Tcl expert, but now that I think about it, I'm not sure you're correct. My recollection is that Tcl is one of the few to have a true hybrid engine. So

I'm not a Tcl expert, but now that I think about it, I'm not sure you're correct. My recollection is that Tcl is one of the few to have a true hybrid engine. So if you use look-around or backreferences, then Tcl could provide an exponential runtime. If that's true, then Tcl would be included in "Nonetheless, every popular programming language (except Go[1]) has included an exponential backtracking regex implementation in their standard library," with perhaps a caveat or special mention. But it's still a categorically different guarantee than what, say, RE2 provides.