Non-receipt of articles completion letter

No proof is required as your details are already updated with ICAI. 

If they ask for it, soft copy is very well enough.

The reprint letter is in itself a letter from their official records. Therefore, In my opinion, you won't need any physical letter that your training has been completed. But still, if you require so, you can call their articleship section and ask for the confirmation from their records.

Need not to worry about the physical letter, it might have been misplaced during courrier. 

Additionally, ICAI sends two copies of the same letter - 

1. Student's Copy - To your address
2. Principal's Copy - To your office address

So, You can try asking at your office whether they have received any such letter or not.

https://112.133.194.253/REprintletter/reprint.aspx