Singapore transfer is possible. It requires a single itinerary on approved airlines like SQ and Scoot. Otherwise, to go to the ticket counter, you must have all the requirements to enter Singapore. Same as Thailand but Singapore does not require covid testing or $50,000 USD insurance to transit. The Singapore authorities wisely abolished the onerous (and burdensome) transit PCR test requirements last month.
“The new normal” Does nothing to protect from a virus and probably permanent. At Narita airport now you are not allowed to change terminals (there are 3) even if you have a confirmed ticket. We had an issue with one of our Business class customers last year.