From the latest “Bits from the Release Team” message, it appears that Debian 9 will probably defer Secure Boot support to later.

Secure Boot
At a recent team meeting, we decided that support for Secure Boot in the forthcoming Debian 9 “stretch” would no longer be a blocker to release. The likely, although not certain outcome is that stretch will not have Secure Boot support. We appreciate that this will be a disappointment to many users and developers. However, we need to balance that with the limited time available for the volunteer teams working on this feature, and the risk of bugs being introduced through rushed development. It’s possible that Secure Boot support could be introduced at some point in stretch’s lifetime.

Full message:
https://lists.debian.org/debian-devel-announce/2017/04/msg00013.html
https://wiki.debian.org/SecureBoot
https://wiki.debian.org/UEFI