There doesn't appear to have been any official announcements as of yet. However, I have heard through the grape vine that recently accepted founders with Canadian corps are being asked to re-domicile. You can also see the change by comparing the wayback machine post to the currently available version to see the removal of Canada. Hopefully we get more transparency from YC soon.
It appears that some time in November, Y Combinator changed its investment eligibility to only US, Cayman and Singapore companies. As linked in the Way Back Machine post, they had previously accepted Canadian companies but subsequently removed them from consideration. Has anyone seen any official news statements or posts considering this or has it simply been a sneaky move for them to discontinue investment in Canadian companies? Also open to any insight as to possible reasons for the change (e.g. taxes, tariffs, immigration, etc...).