object-format-disclaimer.txt

Note: At present, there is no interoperability between SHA-256
repositories and SHA-1 repositories.

Historically, we warned that SHA-256 repositories may later need
backward incompatible changes when we introduce such interoperability
features. Today, we only expect compatible changes. Furthermore, if such
changes prove to be necessary, it can be expected that SHA-256 repositories
created with today's Git will be usable by future versions of Git
without data loss.