Singapore confirms that it will apply the EU/Singapore FTA to the UK during the transition period, following the EU notification letter to this effect.
Here's the EU letter to Singapore, sent yesterday. This is a standard letter sent to all non-EU countries. 
Here's the annex to the EU letter to Singapore, setting out the legal details of the transition period. Again this is a standard text. It is updated from the original draft of this annex to take account of membership extensions.
Singapore's reply to the EU. Note that they treat the exchange of letters as an agreement. The VCLT says that treaties can be concluded in simplified form, so this is fine.
Since the UK is leaving with a withdrawal agreement, during the transition period trade with non-EU countries takes place on the basis of that, including the notification letters sent out by the EU. Remains to be seen if any countries reject the letter.
