Passly uses the Graph API to federate Microsoft 365 domains.
From the Microsoft Graph point of view, there are limitations that currently exist in Microsoft Graph PowerShell since some functionalities are yet to be built in.
The following table will show you the functionalities that are already in place and have a corresponding functionality in Microsoft Graph: Find Azure AD and MSOnline cmdlets in Microsoft Graph PowerShell | Microsoft Docs. Get-MsolDomainFederationSettings and Set-MsolDomainFederationSettings doesn't have a replacement yet and customer needs to stay tuned to the Cmdlet Map and the Microsoft Graph Changelog in order to receive updates on when these functionalities are going to be implemented.
However, Microsoft is encouraging customer to start migrating scripts with the functionalities that are already implemented. At the moment only the Set-MsolUserLicense Cmdlet has been deprecated from the MSOnline PowerShell, all other Cmdlets are working and continue to work until properly announced. Please, refer to the following documentation to see the current status of the full list of commands in the MSOnline Module.
If you have other questions regarding the functionality of MSOnline PowerShell Federation commands or other substitution options, then I recommend you open a case to the AD FS team in Azure for guidance, following this support path: Azure\Azure Active Directory Sign-In and Multi-Factor Authentication\AD FS (Active Directory Federation Services).