Infosys denied me said letter and I submitted a LoE with this denial and former supervisor’s informal letter and other official letters. My PR went through.
After @mrandmrs 's suggestion many weeks ago, I contacted Infosys HR on a different email id just for fun and asked for a reference letter, and voila! they gave me a template, I filled it and they got my former manager to verify me and now I have Infy’s reference letter. Almost a year after I got my PR! (maybe I can use it later). Try all avenues open to you. Get whatever you can - like the letters you mentioned - to support your NOC description and throw it in your LoE if you’re unable to get an official letter.