MaD2 on the web was not free at any point, but free to play with ads.
It's a very specific distinction that might not feel all that different to you, but it is what made the development and years of updates of MaD2 possible in the first place, as Kongregate shares ad revenue with devs. MaD2 would not exist without that ad revenue, which has now essentially hit a brick wall with web Flash end-of-life.
MaD2 download exists so people who like the game can keep playing it, while also supporting its development.
If you like the game and would like to see even more content, support it. It's that simple.
Under rating threshold (hideshow)
Regarding another comment I believe there was a way to play Flash end-of-life without it stopping you. the place I believe I found dit from was Newsground where you just simply change your date before 2020 December and it should work.
Supernova player is not able to run MaD2 currently for whatever reason, and it's up to them to fix it since they don't provide any kind of compatibility info or documentation for developers. https://www.kongregate.com/forums/9794/topics/513429#posts-13230330