Regarding hiding depression during application I had dropped out of a masters program because of depression in 2015. I free-lanced for a few years, co-founded a tech. startup and my depression diminished, but then I met with an accident in 2017 which gave me chronic back pain. Bed-ridden for months, I converted my full-time work to part-time, and also started working on my Masters thesis. I defended it successfully a few months back (i.e., in 2019). My back pain also has now decreased significantly. As an aside, my back pain was found to have a correlation with my mental health. It started alleviating after I stopped all medication and started living a normal life, with regular exercise routine.

Now, I wish to go back to academia and am applying for Masters/PhD programs. In my Statement-of-purpose, I would have to mention some reason for dropping out of my previous masters program, because it would be glaringly visible.

My question is: How ethical would it be to not mention my depression at all, and instead substitute it with my accident which caused the back pain? That is, if I mention that I met with the accident in 2015 instead of 2017, which caused me to drop out, would it be "right"?

I am thinking of this because as per this, this and this Academia.SE posts, sharing your depression (to your prospective supervisors) is usually not a good idea. And in my case, while the depression lingers beneath and raises its head once in a while, it is usually not present. So, if I am free of depression (but, definitely still "prone"), then should the history be mentioned?
You are under no obligation to share your medical history in an application, but you are under obligation to be truthful -- so no lying about the date of your accident.

It seems like you actually made the best out of your decision to drop out of the program back then. It's legitimate to say, in an application, that you felt at the time that the program was not the right fit for you, and that you wanted to pursue other opportunities -- which you could then describe by saying that you founded a start-up and worked on your MSc project on the side. If you phrase this right, it actually looks quite good, and it's not even a lie: You did leave to pursue other opportunities.