The previous BasicCookieStorage did not enforce RFC 6265 scoping rules around cookie domain, path, and Secure attribute. A client using a single storage instance to talk to multiple origins could have cookies leak across domains, or have Secure cookies sent over plain HTTP — the deprecation message states it bluntly: "BasicCookieStorage has no domain/path/scheme scoping and leaks cookies across origins. Use DefaultCookieStorage instead."
Who is affected: any client using BasicCookieStorage directly with cookies for more than one origin or scheme. Single-origin uses are unaffected.
The fix introduces DefaultCookieStorage (RFC 6265 compliant) as the drop-in default; BasicCookieStorage is renamed InsecureCookieStorage and remains available for callers with a deliberate need for the old behaviour.
Workarounds
For deployments that cannot upgrade immediately:
Use a dedicated BasicCookieStorage instance per origin / scheme, or
Switch to a separate RFC 6265-compliant cookie store implementation.