Get list of inactive users with licenses assigned in Microsoft 365

To be more precise you could:

1/ Get all the users with licenses assigned
The most simple solution is: Get-MsolUser -All | where {$_.isLicensed -eq $true}

To go further, I use several custom scripts :
- To get a list of the licenses assigned to a user (ex : Office 365 E3) : https://github.com/thijoubert/Sharing/blob/main/PowerShell/AAD_Licenses_Users.ps1
- To get a full list of the SKU assigned to a user (ex : SharePoint Online P2) : https://github.com/thijoubert/Sharing/blob/main/PowerShell/AAD_Licenses_Users_Details.ps1

2/ Get the last sign-in date for you users:
The most simple solution : https://docs.microsoft.com/en-us/azure/active-directory/reports-monitoring/howto-manage-inactive-user-accounts
You could use Graph explorer with an admin account

3/ Cross-reference the two lists to get your answer