Yes, Relieving & Exp.Letter are mandatory doc to submit when you join any new org.
Since it's been 7 months, you haven't received your separations letters, i would suggest to drop email to concern HR dept. keeping your manager in CC.
If aforesaid is already done and no response , then you should visit office once to understand the rationale or you may also loop higher authorities.
Also, if they have released you on the same date as of your resignation, you are entitled to get Salary inlieu of notice period as per the clause mentioned in your appointment letter.(do check)
<Provided, you have not requested early release and you were ready to serve entire notice period>
From India, Pune
From India, Bangalore