https://www.bungie.net/en/Explore/Detail/News/50543
In no way shape or form have Bungie explicitly stated that buying the seasons will come with Dungeons. Many are worried that their anniversary party is an experiment to see if the community will be willing to pay for dungeons as standalone.
This post and the other are both speculation, but both agree that if you buy all the seasons then you should get access to the dungeons, and Bungie should not lock it behind yet another pay wall.
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