Following snippet attach an ModifyEvent to each worksheet in a workbook
when document opened :

Option Explicit

Global NumSheets as integer
Global SheetsEvents() as Object

Sub OnDocumentLoad  'ATTACHED TO DOCUMENT OPEN EVENT

  dim iSheet as integer
  dim aListener as Object
  
  NumSheets = ThisComponent.Sheets.Count
  
  ReDim SheetsEvents(NumSheets)
  
  for iSheet = 0 to NumSheets-1
    aListener = CreateUnoListener("Sheet_", "com.sun.star.util.XModifyListener")
    ThisComponent.Sheets(iSheet).AddModifyListener(aListener)
    SheetsEvents(iSheet) = aListener
  next iSheet
End Sub

Sub Sheet_Modified(Evt as object)
  MsgBox "Sheet modified"
End Sub

Sub Sheet_Disposing(Evt as object)
End Sub

If I have for example these 3 worksheets in document : "Table1", "Table2" and
"Table3", deleting worksheet "Table3" doesn't kill the event handler but does
attach it to "Table2" worksheet, making the event handler called twice when
modifying "Table2" worksheet.
Even whorse, undoing the deletion restores "Table3" sheet but the event handler
still remains attached to "Table2" sheet.