We have two places that check the flag, the writeback tracking
code:

block/blk-wbt.c:#define REQ_HIPRIO      (REQ_SYNC | REQ_META | REQ_PRIO)
block/blk-wbt.c:        if ((rw & REQ_HIPRIO) || wb_recent_wait(rwb) || current_is_kswapd())

and bcache:

drivers/md/bcache/request.c:        !(bio->bi_opf & REQ_PRIO))
drivers/md/bcache/request.c:        !(bio->bi_opf & REQ_PRIO) &&

the original idea was that not all I/O is a priority we should not
blindly boost priority for metadata.  Of course wbt actually ignores
and it and we may just give in and just check REQ_META, except for
the fact that f2fs now actually submits REQ_PRIO requests without
REQ_META..