I wonder what happens to the SPDY-specific directives once support is removed. I use 'listen 443 ssl spdy;' and 'add_header Alternate-Protocol 443:npn-spdy/3;'. If these are just ignored no problem, but I wonder if it breaks anything?
Those directives will no longer pass the linting that nginx does on its configuration file. New versions of Nginx with spdy support removed will not start until you remove those directives.
They are treated as syntax errors, but I think it does give an error message that spdy is deprecated and to remove.