Fans immediately recognized that this was likely a reference to Kamen Rider Heisei Generations FOREVER, the 2018 crossover film between Kamen Rider Build and Kamen Rider Zi-O.
TokuSHOUTsu confirms down-thread that this announcement refers solely to the film Kamen Rider Heisei Generations FOREVER, and not to the Heisei Generations series as a whole or any TV series.
The film is part of the franchise’s Movie War series, and also wraps up the Riders’ Heisei Generation.