What 3D printer filament best imitates rubber with a shore A hardness of 50-60?
There are no filaments that are that soft. The reason is that at that softness, the filament would be like wet spaghetti. It would be impossible to push it through the Bowden tube and the nozzle.
I have tried with filament as soft as 85A and even though I technically got it to work, it was not worth the headaches. It had to be printed at an extremely slow speed (like 10mm/s). The nozzle jammed up repeatedly as well and I had to restart the print many times.
A better option is to use elastic resin. I use a Formlabs Form2 printer and the elastic resin is 50A durometer. I have printed with it many times and it is easy to print. You will need more supports than firmer materials otherwise the part could tear during the printing process.