Go to "Communities" > "All communities"
Filter by subscription status: "subscribed"

Now you can unsubscribe each community with a single click, without having to go into the community first.

In the current stable version of PieFed (which piefed.social is on), there is a button to "Leave all communities" down at the bottom of the all communities page.

If you would like to be more selective, then it is pretty quick to go to filter that list to just communities you are subscribed to, and leave a bunch from that page.

Another option is that the Interstellar app has an option to unsubscribe you from everything. That option is in the Settings -> BETA: Data Utilities -> Account Reset.

In the next version of PieFed, I moved the leave all communities button into the Edit Profile section of the site, next to the account deletion option. I also added this ability to the API to make it easier for apps to implement.

Note: There's a 'mass unsubscribe' button coming to the next release of Piefed.

Not sure if it exists currently but I'd imagine a "View All Subscribed Communities" page where people can deselect from a list to then submit might be handy