The Rotobot plugin uses machine learning to create mats. Using the common deep learning techniques, Rotobot is able to isolate instances of pixels belonging to "semantic" classes of objects like people, cars, etc. In practice, Rotobot knows 81 object types, of which the plugin can separate up to 100 into (and out of) an image.
The plugin is not 100% accurate, but it can be used for fast (and not always clean) hold-out masks. Or to automatically pixelate moving objects in the image.
FXGuide has 
a detailed description of the program online and compares the costs with the potential benefits. You can also get an idea of the possibilities and the quality of Rotobot.
A twelve-month license for the OpenFX plugin starts at 130 dollars. A free watermarked trial version is also available. According to the website the plugin should work not only under Nuke but also under Natron and Resolve. It works under Windows as well as Linux and MacOS.
