I’ve seen pictures of thousands of letters bye the grave (ohel) of the Lubavitch Rebbi.
Is it acceptable practice to write a letter to someone who isn’t alive (asking for some sort of heavenly help)?
Thank you for your question.
Yes it is common practice for people to leave letters at the graves of Chasidic Rebbes and tzadikim. It is a form of making a request or a request that the soul of the tzaddik should pray n the person’s behalf. Thee is also a common practice that when people come to the Western wall to place such papers indies it, as a form of praying or writing a letter to G-d with a request.