SQL Statement to query a Power BI Dataflow

You should be able to add a filter step in your query editor to select just 2021 data. I don't think the dataflow necessarily uses SQL but that shouldn't matter.

Your query will look something like this with the new step.

let
    Source = PowerBI.Dataflows([]),
    #"xxxxxxxxxxxxxxxx" = Source{[workspaceId="xxxxxxxxxxxxxxxx"]}[Data],
    #"yyyyyyyyyyyyyyyy" = #"xxxxxxxxxxxxxxxx"{[dataflowId="yyyyyyyyyyyyyyyy"]}[Data],
    #"Sales" = #"yyyyyyyyyyyyyyyy"{[entity="Sales"]}[Data],
    #"Filtered Rows" = Table.SelectRows(#"Sales", each [sales_date] >= #date(2021, 1, 1))
in
    #"Filtered Rows"