Yes, seems this is a 'feature' with TinyMCE 5 on Tokyo - but agreed major downgrade to user behaviour! 

Response from ServiceNow:

Hello 

I have looked into the details of the issue and this behavior has been reported as "PRB1609720". However, the development team confirmed, this is working as expected because of the version of TinyMCE being used now.
I have attached the problem in the related records section of the Case for your reference.

Starting from TinyMCE version 5 we have to use control + right click to enable the context menu.
Below is the direct link to KB which mentions the same :
https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB1189657

We are placing the Case in Solution Proposed state with this information. Please review this and let me know if you have any questions and we are glad to assist you further. If everything is good please click on 'Accept Solution' to close the case.