Well luckily for you, and I am sure others, Firefox will allow you to re-enable this in your browser once version 86 rolls out. As cited from the article:

Set the browser.backspace_action to 0 in the about:config settings panel to re-enable support for the Backspace key as a Back button.

It's not being removed, the default is being changed. You can still change it, just as you have been able to for some time. I support your ability to use this feature, and I also enjoy firefox because of its customization support, but I think it's reasonable to change the default to match what most people would expect.