MS Bookings

AFAIK, there is no single Graph endpoint today that gives you “last activity” for all Microsoft Bookings calendars tenant-wide, but I believe you can get very close with a Graph + PowerShell approach that can be used for cleanup.

If I read your post correctly, you want to enumerate all Bookings in the tenant and then query appointments in the last 90 days and then flag those with zero activity as candidates for decommissioning.

I think this should do what you want:

Connect-MgGraph -Scopes "Bookings.Read.All","User.Read.All"
$businesses = Get-MgBookingBusiness -All
$cutoff = (Get-Date).AddDays(-90)

$results = foreach ($biz in $businesses) {
    try {
        $appointments = Get-MgBookingBusinessAppointment `
            -BookingBusinessId $biz.Id `
            -Filter "start/dateTime ge '$($cutoff.ToString("yyyy-MM-ddTHH:mm:ssZ"))'" `
            -Top 1

        [PSCustomObject]@{
            BusinessName = $biz.DisplayName
            BusinessEmail = $biz.Email
            Created      = $biz.CreatedDateTime
            HasActivity  = ($appointments.Count -gt 0)
        }
    }
    catch {
        # Orphaned and test businesses will likely error
        [PSCustomObject]@{
            BusinessName = $biz.DisplayName
            BusinessEmail = $biz.Email
            Created      = $biz.CreatedDateTime
            HasActivity  = $false
        }
    }
}

$results

You might also want to check the shared mailboxes used by Bookings, which can also be an indicator of activity. This script can help you do that.

Connect-ExchangeOnline
Get-Mailbox -RecipientTypeDetails SharedMailbox | Where-Object {$_.DisplayName -like "*Bookings*"} | Get-MailboxStatistics | Select DisplayName, LastLogonTime

No appointments + no mailbox logon in 90 days is a strong candidate for decommission. If the mailbox is active but there are no appointments, you can review it manually.

Hope this helps!

https://github.com/MicrosoftDocs/microsoft-365-docs/blob/public/microsoft-365/bookings/best-practices-shared-bookings.md#best-practices-for-using-shared-bookings

We’ve seen this come up in larger tenants with lots of “test” or inactive Shared Bookings calendars. A practical approach is to combine Microsoft Graph Bookings data with Exchange Online mailbox telemetry to determine true inactivity over a 90‑day window, then decommission with a short, well‑communicated grace period.

Recommended steps (high level):

Inventory Bookings businesses and check appointment activity within the last 90 days to identify calendars with zero usage.

Corroborate with scheduling mailbox signals (e.g., last logon or item growth) to avoid false positives from seasonal services.

Governance: notify owners, set an opt‑out deadline, and remove or disable unused calendars; keep an exception list for seasonal departments.

References:

Turn Bookings on or off (tenant‑level controls, scoping, and governance):

https://learn.microsoft.com/en-us/microsoft-365/bookings/turn-bookings-on-or-off?view=o365-worldwide

Bookings in Outlook (behavior and user experience differences that affect monitoring):

https://learn.microsoft.com/en-us/microsoft-365/bookings/bookings-in-outlook?view=o365-worldwide

Microsoft Graph Bookings module (endpoints and cmdlets to enumerate businesses and query appointments):

https://learn.microsoft.com/en-us/powershell/module/microsoft.graph.bookings/?view=graph-powershell-1.0

After you identify candidates with no activity in the 90‑day window, send a final notice to owners with a removal date. Anything without a response by that date can be safely decommissioned, while documented exceptions remain active.

Send out an all user email that states, "IT will be removing all Bookings calendars unless you notify us by X date. Please note if we don't hear back from you we will conclude you no longer need the calendar and it will be permanently deleted after that date." 

Making the users responsible for their items is a good thing, not bad.