Would it be bad for Mac to use older printer drivers not meant for that OS? I have old printer and in Canon website, there are drivers, latest for OS Lion, not for El Capitan. 

Would it be dangerous to try and install drivers meant for Lion? Or would the issue just be if it works or not?
I'm just wondering if there is more to dividing drivers by OS than incompatibility or would it cause security risk?
Or would the issue just be if it works or not?


Most-likely this. Since 10.7 was released, there have been some big changes to the rules for macOS drivers (kernel extensions, or kexts).

The two biggest changes are:


Kernel extensions must be signed (unless you disable the signature check, which is enforced by System Integrity Protection)
3rd-party extension now need to be in /Library/Extensions, rather than /System/Library/Extensions.


Most-likely System Integrity Protection will prevent you from installing the driver, and even if you put the Kext in the right place, it may not be signed properly or even compatible.

So it's probably not even possible.