>CONFIG_BLK_WBT=y
># CONFIG_BLK_WBT_SQ is not set
>CONFIG_BLK_WBT_MQ=y
So writeback throttling is enabled only for multi queue devices in your case. I suppose you need to use blk-mq for your sd* devices to activate writeback throttling (scsi_mod.use_blk_mq=1 boot flag) or to recompile kernel with CONFIG_BLK_WBT_SQ enabled.
>CONFIG_BLK_WBT=y BLK_WBT_ MQ=y
># CONFIG_BLK_WBT_SQ is not set
>CONFIG_
So writeback throttling is enabled only for multi queue devices in your case. I suppose you need to use blk-mq for your sd* devices to activate writeback throttling (scsi_mod. use_blk_ mq=1 boot flag) or to recompile kernel with CONFIG_BLK_WBT_SQ enabled.