This article from the Rail Journal says:
The Secretary of State of the Dutch Ministry of Justice and Security, Ms. Ankie Broekers-Knol, informed the parliament of the country on September 25 that an agreement had been reached between the Netherlands, Belgium, France and Great Britain on customs arrangements and security for Eurostar services in Amsterdam – London. .
Direct services will begin operating between Central Amsterdam and London St Pancras on December 15, when the 2020 calendar enters into force.
However, there has been no Eurostar press release on the subject, and the Eurostar website continues to show only trips with a change in Brussels.
I am afraid that the deadline for the 2020 time change may have been missed, so this would probably delay things for the next time change, probably the summer service that should take effect in June. Eurostar only shows trips until May 13, so it is too early to secure it.
The Bahn.de shows direct trains on some dates (from March 31 to May 16, with some of the trains that do not work on Saturdays and / or Sundays), but does not allow you to book them.
The Oui.Sncf site shows direct trains, but says "Train non ouvert", which means that the train cannot be booked yet (but you can subscribe to an alert to receive a notification when they are available, although I am not 100% sure that works in this specific case).
I guess the trains are planned to work as direct trains, but they are not quite sure if they can really work as such, depending on the completion of the work in Amsterdam CS and / or Rotterdam CS, and they will not. Allow these trains to be booked until they are sure they will work as planned.
Interestingly, this other Rail Journal article, published later than the first, states
NETHERLANDS Railways (NS) has announced that the frequency of London-Amsterdam Eurostar services will increase from three per day currently to four per day in 2020 and five per day in 2021.
However, the gradual expansion of the service will only occur if a four-country treaty is signed, involving Great Britain, France, Belgium and the Netherlands, in relation to border controls.
NS says it expects direct services Amsterdam – Rotterdam – London to begin this spring.
I am not sure if the reports are accurate and if the problem is that the agreement has been "agreed" but not really signed, or if only the completion of the work required in Amsterdam CS and Rotterdam CS is pending. Given the tweet linked below, probably only the last one.