To be done after 1.17-stable is branched.

Scan the codebase for deprecations marked with a 1.18 deadline and remove them (ack deprecation.*1.18). All deprecations with a 1.18 deadline have been shipped (at least) in 1.16 and 1.17.

Marked for a 1.17.0 release so it's done at the time of 1.17-stable branching, but please move to 1.18.0 when done.