Semi-additive max date

So something like this?

Customers Lost =
VAR EndDate = DATE ( 10, 30, 2021 ) // Or from filter context
VAR MaxChangeDates =
    ADDCOLUMNS (
        SUMMARIZE ( Subscription, Customers[Name] ),
        "@MaxChangeDate", CALCULATE ( MAX ( Subscription[ChangeDate] ) )
    )
VAR AddEndDateCol =
    ADDCOLUMNS (
        MaxChangeDates,
        "@MaxEndDate",
            CALCULATE (
                MAX ( Subscription[EndDate] ),
                TREATAS ( { @MaxChangeDate }, 'Date'[Date] )
            )
    )
RETURN
    COUNTROWS ( FILTER ( AddEndDateCol, [@MaxEndDate] = EndDate ) )

( ^^ Not tested but hopefully gives an idea for the logic, at least.)