I certainly can't see any advertised.
Last time I think they just reorganised the services from St Pancras with 395s to approach Sandwich via Canterbury West and Dover. I suspect something similar will happen this time.
Golf isn't really the sort of event for special trains as it takes up the whole day so there isn't much time left in the day for charters with dining etc to make sense. However, looking at sixbellsjunction, I note that they did run the VSOE 'British Pullman' and 'Northern Belle' carriages (to Deal) in 2011. That isn't happening this time as both trains appear to be in use for different trips on the relevant days.
In contrast to what (sometimes) happens in Scotland, Southeastern (just about) have rolling stock they can divert to cover the peak loads so no need for charter stock to be used for the masses.
From last time
https://www.bbc.co.uk/news/uk-england-kent-14183487
Network Rail seem confident of their station work being able to handle the passengers and refer to 12-carriage trains.
https://www.networkrail.co.uk/running-the-railway/our-routes/kent/sandwich-station-improvements/#:~:text=Sandwich Station improvements completed ahead of The 149th Open in July 2021&text=The 149th Open will be,of the event in 2020.