How can I add to Ukrainian keyboard/layout a backlash symbol

aobiednikov wrote:

How can I add to Ukrainian keyboard/layout a backlash symbol?

I couldn't find a way to replace absolutely useless ₴ symbol with \ or / . how can I do that?

Which Ukrainian input source are you using? It should have those characters somewhere. To find them, use

Use the Keyboard Viewer on Mac - Apple Support

See also

Apple Ukrainian Hardware Keyboard: Input… - Apple Community

If you want to make a custom layout, try

https://software.sil.org/ukelele/

Have you tried using a shortcut via the System/Keyboard/Text Replacements preference pane? Replace the ₴ with /.