This means cross-origin requests will have referrers (as manifested in the HTTP Referer header and the Document.referrer API) bearing only the requests' initiating origins in situations where these referrers would previously have contained full initiating URLs.
Why does this matter?
Some sites inspect referrers for access control or for analytics. We've received reports of isolated breakage while rolling this change out. Depending on the embedding product, testing or developer outreach might be appropriate to understand or mitigate the compatibility impact.