A method for implementing digitigrade legs into ChilloutVR / VRChat that results in realistic bending of joints, despite all systems between Unity and CVR / VRC squarely built for plantigrade legs and absolutely nothing else.
Works wonderfully in fullbody, and in my own test case, works even better for making the default animations look nice than it does making the fullbody better. And now with this methodology, this doesn't require anything other than what Unity provides, and is CVR / VRC whitelisted of course.
If you're having issues with the tutorial and you're not using Blender, the 2.1 version should be more software agnostic. (Use the Unity setup from this guide though, that one is now out of date.)
If you want this, but don't want to do the leg work yourself, I can do it for you. (Pun 100% intended.)

VIDEO VERSION by KaideArt
ホネトリ氏による日本語版 (2) (3)
Advanced Edition For if you understand how this setup works, and wish to optimize things a little farther at the expense of the straightforward simplicity of the primary tutorial.

# Minor note about needing the hip-knee and ankle-foot sections to be nearly or exactly parallel to each other for this to work. This is the sort of oversight you have when you get blinded by how easy it was the first time. Live and learn I guess.

# Re-did Section #1 to hopefully make it more fool proof for explaining what needs to be done. At the loss of the elegance of the original description for doing it.

# Very minor changes to hopefully negate some ambiguity.

# Turns out, Aim Constraints were never necessary at all. On the digi-ankles now there's a Rotation Constraint that's constrained to the planti-hip. Completely fixes any ankle-flipping at impossible angles and leg stretch. Thanks again to Manyblinkinglights for pointing this out.

# Relented and put in a new section about how to solve the "IK lock", with the trade off of it breaking 1:1 foot tracking when bringing the foot back. Ideally, the legs of a given avatar would be made / modified with ideal proportions, but in the case of wanting a specific aesthetic or some other technical reason things have to stay, there's only so much you can do to make a real human leg map to a non human leg. THIS IS NOT TO SAY IT'S NOT FINE 90% OF THE TIME. Just it is a issue that crops up.
Also a section about said ideal proportions.

# Made it less VRChat specific given the sudden mass interest in ChilloutVR, added a note about the difference between CVR and VRC (Spoiler: ChilloutVR requires a toe bone to be set up, VRChat doesn't), and a couple of minor addendum's here and there.

# I was mistaken about the necessity of toe bones for uploading avatars to ChilloutVR, removed that mention as it's straight up false.
# Unnumbered as this was a very minor change, turns out setting the "Lower Leg Twist In-Out" to 0-0 in CVR caused the foot to not track properly to the foot tracker, as the foots ability to twist is tied to the lower leg's ability to twist.