As you know, SPO Communication Sites allows to create a special type of page called Post News that provide a comments area where any user can type comments about the post. But, where are these comments actually stored? This a great question and the answer is that page comments are stored in a hidden list you can discover by using SharePoint Designer 2013 (SPD 2013):
If you browse list details in SPD 2013, you will see that the list is storing comments information:
But here is where I have found a great challenge: so far I have not been able to access to comments information stored in the list no matter if I use a tool to easily access this information (CAL Designer or SharePoint Online Client browser) or some PowerShell coding.