I asked one of the copter's engineers, Matt Keennon, what the hole in that one leg's foot was for. He replied that it's only for stowage.
... because of the odd way the copter is held in place under the rover, with all sorts of mechanical constraints from the rover, that one leg could not be held down by a simple lever pressing on the leg strut, so instead it was held lower down on the foot. The bar on that one foot can be thought of as little extra bit of leg 'ankle' for the stowing system to hold onto.
1:00-1:11 of a NASA video shows the legs unfolding one at a time as the rover releases the copter. Ben Pipenberg followed up with Matt:
...at 1:07 you can see that the closest foot on the left side extends up into the rover belly pan an inch or so. That foot needs to fold up in a way that it doesn’t overhang the sample cache system when it’s stowed, avoids a hole in the belly pan for a pyro cutter installation step, and stays within the not-to-exceed volume for the heat shield. As a result we had to cut a hole in the belly pan to make the geometry work out when the helicopter is deploying. Well, on the other foot there’s other rover hardware in the way ... so we can’t just cut a big hole there-- we needed to move that foot out a few inches, and the easiest solution was to add a handle onto the foot that lets the MHDS system grab it.
In https://mars.nasa.gov/system/downloadable_items/44764_PIA23823-HelicopterCloseUp.jpg the leg closest to the camera is held by its ankle, but the leg pointing to the right is held by the hole in its foot.
Another view of Ingenuity stowed under Perseverance belly shows a strut hold through that hole.
Image source: https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA23824