The easiest way to do this is to use SFDX/VSCode (or some other similar tool that interfaces with the Metadata API). Once you have everything set up, you'll be able to pull down your org's Metadata. Then it should be as simple as CTRL/CMD-F to see if the template name is referenced in any files.
I think you can not call JS from a button in Lightning.
When transitioning from classic to lightning one of the things you should take care is exactly that.
SF considers calling JS from a button to be security risk and hard enforce good coding practices there.
You should design the same functionality using hopefully Process or Flow.
If they are not ...
I'm working on similar solution. It's working as expected for me. Below is my URL hack
Since you want to bypass the custom component override for these URL buttons, you can do this by simply adding a nooverride=1 parameter to the URL
Adding this parameter will bypass your custom override, leaving the standard new buttons pointing ...
You can handle this by using onkeypress and checking that the pressed key is 13.
Unfortunately, what you are looking for is not a supported feature in Lightning at the moment (see idea link below). This is a painful limitation on the platform, but is not totally hopeless as you can still create list buttons and add to your lightning pages. The limitation here is that you have to decide whether to send the user to a URL, run some ...