Update (May 2023):
You can use Get-PnPAzureACSPrincipal to returns the lists of all Azure ACS principals installed in your Tenant including subsites.

Scenario

Developers in the organization can use both – Azure Apps and SharePoint Apps to work with SharePoint sites in their “daemon” applications.

It is recommended to use Azure apps so, you want to know – what are SharePoint Apps registered and their owners, who registered SharePoint Apps. Eventually you would disable SharePoint Apps-only principal but before that you’d move Devs from SP-App-only to Azure App (see Disable Custom App Authentication).

(SharePoint App-only service principals aka SP-App-Only are SPN or App registered from within SharePoint using AppRegNew.aspx system page).

One of the approaches is to track Apps/Owners with Unified Audit Log

Use Unified Audit Logs

The following PowerShell code:

$operations = 'Add service principal.'
$recordType = 'AzureActiveDirectory'
Search-UnifiedAuditLog -StartDate $start -EndDate $end -ResultSize $resultSize -Formatted -Operations $operations -RecordType $recordType

returns events with operation = ‘Add service principal.’ Nice, but…
if an app was registered in Azure – event contains an UPN under UserIds property:

Unfortunately, in case with registering app in SharePoint, an audit log event will be like:

i.e. UserId registerd is “spo_service@support.onmicrosoft.com”, so we do not know who registered a SharePoint-only app

In theory – we could use events recorded immediately before and after “Add service principal” event to track a user and site who has registered a SharePoint-only app… But for me it seems like too complicated for automation.

Instead we can do simple search through audit log for events “AppRegNew.aspx page visited”. This gives us a good approximation of who registered SP-App-only principal. Worst scenario – we reach more people than we really need (including those who started registering sp-app-only but did not complete) but all of them would be definitely our target auditory.

Consider the following code:

$freeText = "appregnew"
$operations = 'PageViewed'
$recordType = 'SharePoint'

$results = Search-UnifiedAuditLog -StartDate $start -EndDate $end -ResultSize $resultSize -FreeText $freeText -Operations $operations -RecordType $recordType 

this would give you all users who loaded “/_layouts/15/appregnew.aspx” page

Update: Sites.Selected API MS Graph permissions was introduced by Microsoft in 2021. It was a good move towards site-level development, but still developers were limited with only what MS Graph API provides for SharePoint dev.
So devs had to use AppInv.aspx at site level to provide ACS permissions to their apps to be able to use SharePoint CSOM and REST APIs.
Recently Microsoft introduced Sites.Selected SharePoint API permissions for registered Azure Apps! So now devs should be fully happy without ACS-based permissions.

References

• Export all SharePoint App Principals using the Microsoft Graph PowerShell Module
• Use a PowerShell script to search the audit log
• PowerShell samples to get Apps registrations from Unified Audit Log
• Azure App, ACS, SP-App only and Disable Custom App Authentication
• Accessing SharePoint using an application context, also known as app-only