Season 29 will be the last Season to add new content to Diablo 3, as reported by Forbes.
Every Diablo 3 Season brought something fresh to the game with its unique themes and mechanics. For example, the latest Season 28 introduced a character progression system called Altar of Rites.

However, this journey will come to an end soon with Season 29, the last content update for Diablo 3.
Beginning with Season 30 and beyond, Diablo 3 will enter maintenance mode where you can still play it, but there will be no new content. New seasons will essentially mix themes and cosmetics from old seasons.
Source: Forbes