All the rows after last "8" in PQ

I guess you need table with values after last 8 in Numbers, not column with above values and null before.

Not sure simple way with UI only exists. I'd modify a bit what OliverScheurich  suggested by using Index, not Number in conditional column.

• Add Index column
• Add Conditional Column (if Number equal 8 then Index)
• Fill this column down
• Group By this column without aggregation
• Sort result descending and keep first row
• Keep only column with Data
• Drill table down
• Filter on first row with number 8
• Remove all columns but Number

With M-Code it could be much easier

let
    Source = Excel.CurrentWorkbook(){[Name="Tabelle1"]}[Content],
    Result = Table.RemoveFirstN( Source, List.PositionOf( Source[Number], 8, 1) + 1 )
in
    Result

An alternative could be Power Query. In the attached file you can add data to the blue dynamic table. Then you can click in any cell of the green table and right-click with the mouse and select refresh to update the green result table.