Digital signatures have in the past few years become more common, and more apps provide a variety of ways to digitally sign. Some stock photo agencies accept different type of apps with digital signatures for model releases. There are different requirements each type must fulfill in order for the identity of the person to be confirmed.
The legislation about digital signatures vary in different countries. Although it is easy to make a quite safe assessment of US and EU digital signatures, it may not be so easy for other countries.
Model Release Validity
Just because the stock photography agency were you submit your photos accept a specific app, does not necessarily mean that specific form of digital signature is valid in the country you use it. If you rely on the model release for additional purposes than stock photography, make an inquiry if the specific form of digital signature is accepted locally.