Found it:
The King's letter in full:
Following the decision by the Duke and Duchess of Sussex to relocate to the UK, we have received a number of requests for Guidance. To help avoid doubt or confusion, The King has directed that the following information be shared.
It is well known that in January 2020 the Duke and Duchess stepped down from undertaking representative duties on behalf of The Sovereign, and are no longer working Members of The Royal Family. This position, distinct from the State and Royal duties undertaken by the working Royal Family, and with the personal latitude it brings the couple in respect of financial independence and protection of their privacy as they would wish, will continue to be fully respected.
It follows that there is no change to the current status of the Duke and Duchess of Sussex. Their styles as His and Her Royal Highness remain in abeyance and are not used. The charitable work of the Duke and Duchess is a personal matter for them both and undertaken in their private capacity. In short, their position is akin to private citizens with commercial and charitable interests.
Any specific questions from official and State organisations about the courtesies which should be extended to the Duke and Duchess, and particularly where recourse to public funds may be required, should be directed to Buckingham Palace. Any operational security issues should continue to be directed to the relevant Police authorities.
All correspondence and administrative questions relating to the couple should be sent directly to their office, which will continue to operate separately from The Royal Household.
The Right Honourable The Lord Benyon GCVO