In an interview with Gamerant, Blizzard revealed that Eternity's End is the final story arc of Shadowlands. A separate interview with Inven confirmed that Season 3 was the final season of Shadowlands.
The Gamerant interview stated the following:
With the stakes this high, it is impossible not to sense the finality of the story. In the history of World of Warcraft, expansion packs have followed a release cycle that consists of three major content patches, which fill the two-year gaps between. Read more on wowhead.com