Example using Microsoft Graph PS Module to query signInActivity/lastSignInDateTime

Get-LastSignInDateTime.ps1

# One time installation

    Install-Module -Name Microsoft.Graph

# connect

    Connect-MgGraph -Scopes "User.Read.All", "AuditLog.Read.All"

    Select-MgProfile -Name "beta"

# will return all users with a date value for lastSignInDateTime

    $date = [DateTime]::Today.AddDays(-90)

    $users = Get-MGUser  -Filter "signInActivity/lastSignInDateTime le $($date.ToUniversalTime().ToString("yyyy-MM-ddThh:mm:ssZ"))" -Select Id, userPrincipalName, mail, onPremisesLastSyncDateTime, onPremisesDistinguishedName, signInActivity

    $users | SELECT ID, mail, userprincipalname, onPremisesLastSyncDateTime, onPremisesDistinguishedName, @{N="lastSignInDateTime"; e={$_.signInActivity.lastSignInDateTime}}



# will return all users with no date value for lastSignInDateTime (has to pull all users, so will be slow)

    $users = Get-MGUser -Select Id, userPrincipalName, mail, onPremisesLastSyncDateTime, onPremisesDistinguishedName, signInActivity

    $users | SELECT mail, userPrincipalName, onPremisesLastSyncDateTime, onPremisesDistinguishedName | ? { $_.signInActivity.lastSignInDateTime -eq $null }