SharePoint Online: Find Unused Lists based on Last Modified Date

Requirement: Find Unused SharePoint Online Lists based on Last Modified Date

How to Get the List Last Modified Date in SharePoint Online?
Go to the "Site Contents" page (https://tenant.sharepoint.com/_layouts/15/viewlsts.aspx) of the site either from left navigation or from site settings menu of the site, which lets you to get last modified date of all lists in the site.

SharePoint Online: Get List Last Modified Date using PowerShell
You can get last modified date value of a SharePoint Online list using PnP PowerShell as,

#Set variables
$SiteURL = "https://crescent.sharepoint.com/sites/marketing"
$ListName = "Documents"

#Connect to PnP Online
Connect-PnPOnline -Url $SiteURL -UseWebLogin 

#Get the List Last modified date         
(Get-PnPList -Identity $ListName).LastItemUserModifiedDate

How about getting the last modified date for all lists in a site? This time let's use CSOM PowerShell script:

#Load SharePoint CSOM Assemblies
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"
 
#Variables 
$SiteURL ="https://crescent.sharepoint.com/sites/marketing"
 
Try {
    #Setup Credentials to connect
    $Cred= Get-Credential
  
    #Setup the context
    $Ctx = New-Object Microsoft.SharePoint.Client.ClientContext($SiteURL)
    $Ctx.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Cred.Username, $Cred.Password)
          
    #Get all lists from the Web
    $Lists = $Ctx.Web.Lists
    $Ctx.Load($Lists)
    $Ctx.ExecuteQuery()

    #Iterate through Lists
    ForEach($List in $Lists | Where {$_.hidden -eq $false})
    {
        #Get List last modified date
        Write-Host $List.Title  "Last Modified: " $List.LastItemUserModifiedDate
    }
}
Catch {
    write-host -f Red "Error:" $_.Exception.Message
}