Is the obligation for posted third-country nationals to notify the UWV still in force?

No, from 1 March 2020 this regulation is no longer in force. From 1 March 2020, you must notify these workers through the online notification portal.

If you post a worker from a third country to the Netherlands for longer than 3 months (under Article 1e of the BuWav), you or your worker must apply to the Immigration and Naturalisation Service (IND) for a temporary regular residence permit in the context of cross-border service provision, in addition to the notification. If the IND receives such a request, it will check whether a notification has in fact been made. If you have made a one-year notification for a worker from a third country and you would like to apply for a residence permit for this worker, you must submit a regular notification for this worker through the online notification portal, so that the IND can see the duration for which this worker is likely to work in the Netherlands.

The previous question provides more information about additional requirements for posting workers from outside the EU, EEA and Switzerland.