I have reviewed the code in the v11 patch and it looks good to me.
But in common_dumpall_restore.c there's parseDumpFormat which is common between pg_dumpall and pg_restore ,as per the discussion in [1] thread i don't think we should create a common api ,as discussed in the thread there might chances in the future we might decide that some format is obsolete and desupport it in pg_dumpall ,while support in pg_restore for compatibility reasons.