Nice one-liner to list all users who haven't signed into Entra in the last 90 days excluding accounts who haven't signed in at all :)
Get-MgBetaUser -All -Property SignInActivity | Select-Object UserPrincipalName,@{label="LastSuccessfulSignInDateTime";expression={$_.SignInActivity.LastSuccessfulSignInDateTime}},@{label="LastNonInteractiveSignInDateTime";expression={$_.SignInActivity.LastNonInteractiveSignInDateTime}} | Where-Object { $_.LastSuccessfulSignInDateTime -lt (Get-Date).AddDays(-90) -and $_.LastNonInteractiveSignInDateTime -lt (Get-Date).AddDays(-90) -and -not($null -eq $_.LastSuccessfulSignInDateTime -and
$null -eq $_.LastNonInteractiveSignInDateTime) }