Is it true that now we can only sell/buy SXDT on Ddex & ForkDelta right? no other coins can be traded
Yes. It was there in IDEX before they take the decision to delist security tokens. Some jurisdictions consider dividend paying tokens as security tokens and most of crypto exchanges do not accept security tokens.
SXDT can be traded only at ForkDelta and DDEX at the moment. CEO mentioned that they will introduce their own DEX to trade SXDT and SXUT soon.