What is this thing I got out of my 7 year old hen's foot?

Glad to hear it. The pocket should get smaller over time as long as it doesn't get as full as it did initially. You are doing a great job. Nice observation skills to have caught that before it became a real problem.

Good luck!

Looks like she may have some scaly leg going on too - couldn't hurt to treat her feet with VetRx or the like for a few days.

If you really want to give her the spa treatment, I love giving my girls a nice soak in a warm foot bath with a very mild antiseptic soap (like Dr. Bronner's with tea tree or lavender) followed by a gentle scrub with a super-soft facial brush or toothbrush to help loosen up the old scales. Once I dry their feet off, I apply Vet Rx if there are no open areas or something more like Burt's Bees Res-Q ointment if there are any spots with broken skin. For scaly leg, I repeat the application of oil/ointment once a day for 2-3 days, then maybe repeat the spa day/oil application every week or two until the scaly leg clears up and I see new scales coming in.

FWIW, given your description and pictures, I think the foreign mass was just dirt/poo that accumulated in that weird toe pocket - it doesn't look like it was actually encapsulated under the skin, which is what happens with Bumblefoot. Either way, I totally agree with CMV that now you know the tendency is there, simply keeping an eye on it should be sufficient to prevent any further complications.