AFAICS itch.io does not make a tool available to make an offline backup of one’s purchases. There are official CLI tools available from itch.io, but they seem to be focused exclusively on the developers’ QoL. The user app is good, but it doesn’t provide a way to backup one’s library.
I can see that there have been several threads about this already (e.g. https://itch.io/t/2998166/downloading-purchases-through-the-cli and https://itch.io/t/2213761/easy-way-to-download-all-games-in-a-bundle and https://itch.io/t/2007428/download-all-files-in-a-bundle but also related https://itch.io/t/1982785/unable-to-find-api-for-dowloading-tools and some of the comments in https://itch.io/post/1500360 ).
I appreciate (from that last thread in particular) that there are good reasons to avoid mass downloas, as the hammering on the server would reduce service quality for everybody, but this could be dealt with implementing strong rate-limiting server-side, and offering a client designed for this.