Disable Modern Comments Delete functionality

Mastahh 

As RobElliott said, Injecting custom JavaScript/CSS on modern pages is not recommended.

However, you can inject JS and CSS files into SharePoint Modern pages using SPFx Application Customizer.

SPFx application customizer will be loaded on all existing and newly created modern pages in SharePoint. So, you have to check if you are on correct list/library within your code and then inject the necessary code.

You can also check the current logged in user and his/her permissions and hide the disable button accordingly.

1. Build your first SharePoint Framework Extension.
2. Inject Custom CSS on SharePoint Modern Pages using SPFx Application Extensions.
3. Step By Step Procedure To Add A JS File To All Pages In A SPFx Modern Pages Using SharePoint Framework Extension.

For more information, check my answers at: 

1. What is the best practice for getting JS and CSS files into a SharePoint Modern Page? 
2. Remove logo at top left from SharePoint online hub site? 

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