Catalina has bundled python 3

$ ls /usr/bin/python3 
/usr/bin/python3
$ xcrun --find python3
/Applications/Xcode.app/Contents/Developer/usr/bin/python3

Can a non-brew-installed python 3 (system bundled or official pkg) suffice the depencency of a brew bottle (e.g. imagemagick -> glib -> python)?

I’ve read in brew install --help

       --ignore-dependencies
              An  unsupported  Homebrew development flag to skip installing any dependencies of any kind. If
              the dependencies are not already present, the formula will have issues. If you're not develop-
              ing Homebrew, consider adjusting your PATH rather than using this flag.

Does it mean if python3 presents in PATH (it should by default), I can install brew-bottled imagemagick without brew-bottled python?

I would recommend against it. While it might work, there’s also an option that the configurations homebrew and apple use to compile python are so different that everything breaks in hard to debug ways. If you’re feeling adventurous you can try it and make a pull request to change the formula to use system python though.