Many commenters praising this here but parent selectors have been in discussion almost since the creation of CSS and the repeated conclusion has always been that they're a performance nightmare by definition.
The CSS WG spent quite a lot of time and effort deliberating on a standard which wouldn’t have a significant performance impact. They only support a subset of selectors as arguments, to minimize their complexity.
True in the sense that you mean and alao in pure CS theory terms but also the TFA also links to some serious documentation on practical solutions to many real world performance problems and browser codebases have most of the underlying complexity for other less obvious performance issues. E.g. https://github.com/Igalia/explainers/blob/main/css/has/has-s...
Thanks for the deeplink, I should spend some time digging in.
Though as far a pre-existing complexity in modern engines goes: engine performance has regressed (in absolute end-user terms), and engine conplexity is also a barrier to entry for competing/new engines. So this isn't exactly "good".
What's changed in that regard?