I am not sure why but even stake removed the max bet option and when I asked the support they told me that the option was being abused by people and they did not gave the exact reason but assured that was the case. I tried to use the max bet option on sportsbet.io now as you asked and I actually have that option but it shows the max bet of the balance like I had 3 mbtc in my account so it allowed to make max bet of 3mbtc while earlier the max bet shown was the actual max bet allowed without being based on my balance.

I don't mind either ways though because I do not bet big and never made max bet in my life on any casino  but I would love an explanation on how it was removed or changed the way it used to work .