It's extremely disappointing that, other than Firefox, browsers have refused to implement subgrid[1]. As I understand it, this feature was deferred from the basic grid spec with the understanding that it was still necessary and would be implemented, not so it could consequently be ignored. If Mozilla was capable of implementing subgrid, Apple and Google certainly have the capacity to do so.
> It's extremely disappointing that, other than Firefox, browsers have refused to implement subgrid
The way I understand the situation in Chromium is that they are slowly moving to a new layout system called LayoutNG [0]. The current grid implementation is in the old layout system however and they don't want to spend time extending it just to abandon it a few months later. There's already an ongoing work to support grid (GridNG) in LayoutNG [1] and once that's done, adding support for subgrid should be "fairly straight forward" [2].
[1] https://caniuse.com/?search=subgrid