Apologies for another delayed response, it’s been a bit hard to give another half answer, but I probably have enough to give an end-of-year update:
Strict Mode
We have no official plans to support Strict Mode. It might be possible after we’ve done a full React 19 migration, but I do not see us enabling Strict Mode ourselves for the foreseeable future and it’s not on our roadmap.
React 19
Probably not a surprise, we’ve started on the journey to React 19 
I expect we’ll share a bit more in 2026 as we have a timeline on dropping React 18 support, but I can say officially we’ve done more than just explorations.
I believe all of the Design System Team packages work in React 19 today (though it’s possible ecosystem support is pending a feature gate). We formally haven’t started dogfooding React 19 anywhere, but we’ve had a few apps get a local working branch and fixed everything we saw blocking that. We’re working on involving the wider platform and frontend monorepo now.
I’m not here to promise a timeline, but ecosystem comms are written as one of the first steps when that roadmap is solidified and fully resourced, but so far React 19 feels much smoother than React 18 — mostly statically deterministic and a matter of removing deprecated APIs and unmaintained third-party dependencies rather than changing the way the entire root renders.