After the federal congress passed that bill, there's no doubt that even more national flags were burned than before.
Literal
Federal-congress [subject-が] that bill [object-を] passed result, until-then more-than many national-flags [subject-が] burn-cast-off-passive [certainly-に違いありません].
Two grammar points worth noting: ~た結果 ('as a result of having done ~') frames the preceding clause as a cause whose consequence is about to be stated, and ~に違いありません expresses strong inference ('must be,' 'no doubt'). The verb 焼き捨てる is a compound of 焼く (burn) + 捨てる (discard), a productive pattern where the second verb colors the first — here 'burn up and be done with.' The passive 焼き捨てられた keeps the flags as topic without naming who did the burning.