Microsoft: “You can also select to allow or prevent commenting on modern pages. If you allow commenting, it can be turned on or off at the page level“

By default both
– Allow users to create new modern pages
– Allow commenting on modern pages
are turned on (enabled)

Tenant or SharePoint admin can find settings under
SharePoint Admin Center -> Settings -> Pages

When you are creating a modern site page – there is an option “Comments” turned On by default:

And page with comments looks like:

Comments on site pages (aka modern pages) can be enabled or disabled at each of the levels:
– Tenant level
– Site (aka site collection) level
– Web (aka subsite ) level
– Page level
Here is how it is done:

Level	How it’s done	Who can do it
Tenant	GUI ( SharePoint Administration) or PowerShell	Global Administrator or SharePoint Administrator
Site (Site Collection)	PowerShell	Global Administrator or SharePoint Administrator
Web (Subsite)	PowerShell	Site Collection Administrator or Owner (Full Control rights to web)
Page	GUI (Page Editing screen)	Site Member (Edit right to page)

If commenting on modern pages disabled at higher level – lower level settings do not work. E.g. If you disable “Allow commenting on modern pages” at tenant level (it takes minutes) – the functionality will gone from all modern pages of all sites.

When you switch page comments Off – all existing comments will be hidden (but not deleted).
If you later turn comments On – comments will reappear, including Likes.

If “Allow commenting on modern pages” disabled at tenant or web level – you will not see “Comments On/Off” switch while editing page.
If “Allow commenting on modern pages” disabled at site collection level – you will see “Comments On/Off” switch while editing page, but you will not be able to turn it ON.

PowerShell

When you disable “Allow commenting on modern pages” at tenant level –
PowerShell Object (site/web) property “CommentsOnSitePagesDisabled” will not be changed for any site/web.
You can still with PowerShell set it to True/False:
“Set-PnPWeb -CommentsOnSitePagesDisabled:$false”
but it does not take any effect.

If you enable “Allow commenting on modern pages” at tenant level (it takes ~10 minutes) – the functionality will return to all modern pages and
all webs and sites properties “CommentsOnSitePagesDisabled” will ???.
You can change it with PowerShell:
“Set-PnPWeb -CommentsOnSitePagesDisabled:$false”.

# having Site Collection Admin Permissions:
# disable Comments On Site Pages for a subsite:
$webName = "SubSite_02"
Set-PnPWeb -Web $webName -CommentsOnSitePagesDisabled:$true 
# enable Comments On Site Pages for a subsite 
# (only if comments enabled at tenant level):
Set-PnPWeb -Web $webName -CommentsOnSitePagesDisabled:$false


# having global admin or SharePoint admin permissions:
# site collection:
Set-PnPTenantSite -Url $siteUrl -CommentsOnSitePagesDisabled:$true
# tenant-level Comments:
Set-PnPTenant -CommentsOnSitePagesDisabled:$true # disable
 comments
Set-PnPTenant -CommentsOnSitePagesDisabled:$false # enable comments

# does not work:
Set-PnPSite -CommentsOnSitePagesDisabled:$true

How do I know if if the page is modern page or classic page (PowerShell)?

$list = Get-PnPList "Site Pages" -Includes ContentTypes, Fields
$list.ContentTypes | ft -a
$cType = Get-PnPContentType -List $list | ?{$_.Name -eq 'Site Page'};
$cType.id
$queryString = "<View><Query><Where><Eq><FieldRef Name='ContentTypeId'/><Value Type='Text'>" + $cType.Id.StringValue + "</Value></Eq></Where></Query></View>"  
$modernPages = Get-PnPListItem -List $list -Query $queryString
$modernPages.count
$modernPages | ft -a

How do I know if the page is a Home Page (PowerShell)?

# web object contains relative link to the web's Home Page:
$web = Get-PnPWeb -Connection $siteConnection -includes WelcomePage
$web.WelcomePage

References

– Microsoft “Allow users to create and comment modern pages“

See also:
– Allow users to create modern pages

Note:
We did not discuss “Wiki Pages” or “Web part Pages”, we discussed only “Modern Pages” (aka Site Pages).
I have tested it all personally using Communication sites.
MS-Team (group-based) and standalone SharePoint (no-group) sites – TBP.