Solutions in the code below. Notice that each uses "PreviousStep" as a basis, so these are separate solutions.
let
Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
PreviousStep = Table.TransformColumnTypes(Source,{{"Date", type date}, {"Letter", type text}}),
// 1. Add a column to the original table with the MaxDate for each letter
// "earlier" is just the name of a function parameter; it could as well have been "x" or "MarcelBeug"
AddedMaxDate = Table.AddColumn(PreviousStep, "MaxDate", (earlier) => List.Max(Table.SelectRows(PreviousStep, each [Letter] = earlier[Letter])[Date])),
// 2. Group by letter and get the MaxDate for each letter
GroupedOnLetter = Table.Group(PreviousStep, {"Letter"}, {{"MaxDate", each List.Max([Date]), type date}})
in
GroupedOnLetter