Employer refused to provide experience letter

I got offer to join as a 'Partner' from founders of a Startup company. We decided to share equal profit as it comes. When I joined, they have not provided offer letter and I did not ask for the same. Because thought, I don't need it as I was joining as a Partner. In the duration of 1 year, many time I requested to initiate legal procedure of a partnership, but founders were delaying anyhow. So I came to know, they don't want me as a company's partner. So I resigned and left company. When I asked for experience letter, they forced me to sign NDA first (Non Disclosure Agreement). I refused to sign NDA as they have not even provided me a offer letter and partnership documents yet. Now they are not ready to provide me experience letter but they have provided salary slips instead. They are saying that, its not company's rule, it's a legal law that, the employee who denying to sign NDA, he cannot be a part of the company and employer cannot provide experience letter. Is this really true? Here, My mew employer asking for experience letter. What do I do?