VALUES('Combination'[Requirement]) gives you a table with all the Requirement values in the Combination table.
If you want to exclude certain result from that table, you need to apply a filter to that table.
You have two options:
Option 1:
VAR tbl_values = CALCULATETABLE(VALUES('Combination'[Requirement]),
                'Combination'[Expiration Date] = BLANK() ||
                'Combination'[Expiration Date] < TODAY())
Option 2:
VAR tbl_filtered = FILTER('Combination', 
                'Combination'[Expiration Date] = BLANK() ||
                'Combination'[Expiration Date] < TODAY())
Then you can replace the VALUES in your measure
List of Requirement values = 
VAR __DISTINCT_VALUES_COUNT = DISTINCTCOUNT('Combination'[Requirement])
VAR __MAX_VALUES_TO_SHOW = 90
VAR tbl_filtered = FILTER('Combination', 
                'Combination'[Expiration Date] = BLANK() ||
                'Combination'[Expiration Date] < TODAY())
RETURN
    IF(
        __DISTINCT_VALUES_COUNT > __MAX_VALUES_TO_SHOW,
        CONCATENATE(
            CONCATENATEX(
                TOPN(
                    __MAX_VALUES_TO_SHOW,
                    tbl_filtered,
                    'Combination'[Requirement],
                    ASC
                ),
                'Combination'[Requirement],
                ", ",
                'Combination'[Requirement],
                ASC
            ),
            ", etc."
        ),
        CONCATENATEX(
            tbl_filtered,
            'Combination'[Requirement],
            ", ",
            'Combination'[Requirement],
            ASC
        )
    )
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