The Ass Was Far Away From the Grass
As someone who struggled with attaining squat depth, I've spent years researching different methods, theories, and reasons why my ass just wouldn't hit the grass. It's not that the methods didn't work, but it was sometimes like I was taking the scenic route.
When I was teaching I found that sometimes people would have the range of motion, but there would be no strength in that range. Sometimes it wasn't sustainable and people were needing to use the same mobilization techniques every session. And sometimes the results were simply so slow that people just decided squatting wasn't for them and probably took up Zumba or something.
You've probably seen this exercise before but bear with me. Even I initially overlooked it because I couldn't do it. It's in the application of the movement that the magic happens.
If you have the range of motion, it's one of the most perfect warm-ups there is for opening the hips, knees, and ankles. Moving from leg to leg and playing with foot positions is a phenomenal way to build strength and awareness.
But what if you can't even hit the position? Well, you can use a counterbalance to access a deeper range of motion.
The key is to spend as much time being active in the position as possible to make it more familiar to your body, then ditch the weight and try to recreate it under your own core strength.
Depending on the severity of your mobility restrictions you may need to start with a heavier weight and gradually decrease the load over a few sessions... or you could very well get lucky and have gained that movement permanently.
Make this your go-to movement. Sometimes individual joint mobility drills have their place, but when you're training to move your body as one unit then applying the same methodology to your mobility work just makes more sense.
Plus you're actually going to build muscle with this rather than sitting on a foam roller smashing your calves for half an hour hoping for some degree of dorsiflexion to magically appear.