For web content that uses keyboard shortcuts, is it acceptable to override existing shortcuts such as ctrl+S, or should we try to find new combinations?
Does this have an impact on accessibility?
I don't think overriding default shortcuts is a good idea. I'll give you an example with Discord. I have been using discord for a while and there's a shortcut to mark all servers as read which is shift + esc but the shift + esc on Chrome, opens Chrome's in-built task manager. So now whenever someone will try to use your shortcut not only will the browser execute the function you have attached with the shortcut but also execute the shortcut the browser has in-built attached to it.