How to disable eSignature feature, what data is affected ?

There is no provision to disable eSignature feature once it is enabled for a project. eSignature feature can not be disabled.

Below are a few points that project administrators can keep in mind:

eSignature Based Approval Workflow can only be enabled by a user having "admin" privileges.
While enabliing eSignature feature, an administrator user is informed that this feature cannot be rolled back.