If we don't then most likely means people can no longer login to Wikipedia the normal way. Or they might be able to login manually, but won't be able to upload files to Commons, interact with Wikidata, send in-page feedback to talk pages on mediawiki.org or Meta-Wiki (like for VisualEditor), etc.
It would also mean when following cross-wiki projects like from Wikipedia to Wikidata/Commons/Wiktionary etc the user might not be auto-logged in.
A cookie associated with a cross-site resource at https://login.wikimedia.org/ was set without the `SameSite` attribute. A future release of Chrome will only deliver cookies with cross-site requests if they are set with `SameSite=None` and `Secure`. You can review cookies in developer tools under Application>Storage>Cookies and see more details at https://www.chromestatus.com/feature/5088147346030592 and https://www.chromestatus.com/feature/5633521622188032.
- (July 2019) This feature is available for testing in Chrome 76 for developers by enabling the cookies-without-same-site-must-be-secure flag. (ref)*
Timeline from https://www.chromium.org/updates/same-site:
- Oct 2019: 50% of Chrome Beta users.
- Feb 2020: Enforced for 1% of Chrome stable for 1 week.
- March 2020: Enforced for 10%, 25%, and then 50% of Chrome stable
- April 2020: Enforcement reduced to 0% for Chrome stable. It remains at 50% of Chrome Beta. (Reason: COVID19).
- July 2020: Enforcement will we enabled for Chrome Stable in Chrome 84 on July 14. (news)