To save Flash storage, can we generate a design bitstream without having BRAM initialization since the design not require ROM?
Are you referring to embedded memory? If yes, if you didn't include any memory initialization on your design then it will be default to 0. You may refer to chapter 3.4 of https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/stratix-10/ug-s10-memory....
The bitstream generated is always compress and this will be the smallest bitstream generated when memory initialization is not used.