Q: Why Indomethacin is not advocated any more for the treatment of Dressler's Syndrome?

Answer: 

Indomethacin inhibits new collagen deposition, and thus impairs  the healing process for the infarcted myocardial region. NSAIDs in general are now less advocated in patients with ischemic heart disease. 

Preferred treatment  for Dressler's syndrome is now Colchicine. 

Corticosteroids are still popular with many folks particularly after cardiac surgeries, but the frequency of relapse is high when corticosteroid therapy is discontinued.