What is the replacement for the funtion "documentlist" in 7.6 version.

That expression is from a shared component and was not part of the base product. Depending on your use case there are other methods of retrieving a list of documents for your use. This thread could be useful to you:

forum.appian.com/.../e-99359

thanks but getdocumentsfromfolder has also deprecated. In my case i want to fetch documents from a folder, is there any workaround for the same.

This expression is part of the Custom Content Functions shared component. It is not deprecated, as it is not part of the base product. As mentioned in the description of the shared components record, "...content is free to use with a licensed product, but it is made available as-is and should be validated and tested before production use."