You could use caml query to get the lasted modified item in a library like this:
$list = $Web.Lists.GetByTitle($ListTitle)
$query = New-Object Microsoft.SharePoint.Client.CamlQuery
$query.ViewXml = '<View><Query><OrderBy><FieldRef Name="Modified" Ascending="FALSE"/></OrderBy></Query><RowLimit>1</RowLimit></View>'
$items = $list.GetItems($query)
$Context.load(items)
$context.ExecuteQuery()
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