Comment on rend_service_descriptor_t lies shamelessly
From the rend_service_descriptor_t struct in or.h:
4832 /** Bitmask: which rendezvous protocols are supported?
4833 * (We allow bits '0', '1', and '2' to be set.) */
4834 int protocols : REND_PROTOCOL_VERSION_BITMASK_WIDTH;
...but version 3 is indicated in the same way and checked for throughout the HS code.