Flag if Latest Period

Your goal is to filter down to just the latest period, right? Although it doesn't create a flag column as an intermediate step, that's exactly what I'm doing in the example I linked to. You can add a flag column too but that adds extra intermediate steps.

Here's an example that adds a flag an then uses it to filter:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTLVN9Q3MjAyAjKNTA2UYnUgwmYIYUNTU7CwEapqZGEk1UbGBnBhcyTVBgjVFkjCZkDVsQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, PeriodM = _t, Value = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ID", Int64.Type}, {"PeriodM", type date}, {"Value", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"ID"}, {{"PeriodMax", each List.Max([PeriodM]), type nullable date}}),
    #"Merged Queries" = Table.NestedJoin(#"Changed Type", {"ID", "PeriodM"}, #"Grouped Rows", {"ID", "PeriodMax"}, "Grouped Rows", JoinKind.LeftOuter),
    #"Expanded Grouped Rows" = Table.ExpandTableColumn(#"Merged Queries", "Grouped Rows", {"PeriodMax"}, {"Flag"}),
    #"Replaced Value" = Table.ReplaceValue(#"Expanded Grouped Rows",each [Flag], each if [Flag] = null then 0 else 1,Replacer.ReplaceValue,{"Flag"}),
    #"Filtered Rows" = Table.SelectRows(#"Replaced Value", each ([Flag] = 1)),
    #"Removed Columns" = Table.RemoveColumns(#"Filtered Rows",{"Flag"})
in
    #"Removed Columns"

Here's a simpler version that bypasses that extra work:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTLVN9Q3MjAyAjKNTA2UYnUgwmYIYUNTU7CwEapqZGEk1UbGBnBhcyTVBgjVFkjCZkDVsQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, PeriodM = _t, Value = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ID", Int64.Type}, {"PeriodM", type date}, {"Value", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"ID"}, {{"PeriodMax", each List.Max([PeriodM]), type nullable date}}),
    #"Merged Queries" = Table.NestedJoin(#"Changed Type", {"ID", "PeriodM"}, #"Grouped Rows", {"ID", "PeriodMax"}, "Grouped Rows", JoinKind.Inner),
    #"Removed Columns" = Table.RemoveColumns(#"Merged Queries",{"Grouped Rows"})
in
    #"Removed Columns"