Is the DBS My Preferred Payment Plan (Deposit Accounts) Promotion essentially a 0% interest 12-month loan with zero fees? Is there any impact or any catch like reducing credit score?

Yes. 0% interest free. No catch. Wont affect credit score if you pay your monthly instalments on time.

But you cannot cancel the card within this 12 months. Even if the bank charge you annual fee for CC, you cabnot threaten to cancel. If you do early repayment, theres penalty.