AFAIK (haven't kicked the wheels on macOS 11 beta yet) you can still manually change all the default icons? I do anyway, I always hated Apple's new ones and found a perfect set I liked instead. All default system icons are still just icns bundles at
And could always just be swapped with whatever you want. With the advent of SIP it's now somewhat more involved since SIP needs to be turned off, icons changed, then back on, or else a specific exclusion done just for that directory. So it's harder now to just have an application or scripts to manage it all automatically and make it trivial to swap sets around at will. But if you're just interested in changing things once in a while (or merely after system updates) it's not a huge lift even now. And applications themselves as well as disk icons, specific folders and so on can all have their icons changed same as always.
Not sure if it's possible for default apps (I don't have a Mac right now), but can't you just open a Get Item dialog, drag & drop an alternative icon onto the icon displayed in the dialog to replace it? (& click the icon, press delete to restore)
