io_request_lock has been replaced by several finer grained locks. The lock relevant to lower level drivers is Scsi_Host::host_lock and there is one per scsi host.
The older error handling mechanism has been removed. This means the lower level interface functions abort() and reset() have been removed.
In the 2.4 series the scsi subsystem configuration descriptions were aggregated with the configuration descriptions from all other Linux subsystems in the Documentation/Configure.help file. In the 2.5 series, the scsi subsystem now has its own (much smaller) drivers/scsi/Config.help file.