If you get the Itch.io version, any feature that currently works with the Itch.io version will continue to work until R18 breakes compatability. You will have access to this version for the life of the product on itch.io.
If you get the Patreon version, you will have access to all the Patreon specific features for as long as you are a supporter. Once your support ends, so does your access to the Patreon version of Jav-It.
Note: To be clear, any files you download and decrypt are yours. It does not require Jav-it for continued use.