See T345245: Mitigate phase-out of third-party cookies in Wikimedia production for context and links to relevant documentation.

OAuth relies on third-party cookies and top-level redirects to identify the user (in some cases without user interaction). With OAuth being a very widely used protocol / workflow, it's unlikely browsers will break it anytime soon; it will probably be covered by heuristics-based exceptions.

Nevertheless we should at some point probably double-check if there is any impact and whether any improvements to the flow could/should be done.