Fallout 76 does not rely on the common season pass model and will not have paid DLCs. This was confirmed by Pete Hines to the British Official PlayStation Magazine.
Good news for fans of Fallout 76: According to Pete Hines, Bethesda’s Vice President for Marketing, the upcoming online role-playing game Fallout 76 will be continuously expanded with free content. Therefore, there will be no season pass or paid DLCs.
This was confirmed by Hines, according to gamefront.de, to the British PlayStation magazine.

What we know about the DLCs in Fallout 76
Regular updates planned: New content is expected to regularly expand and enhance Fallout 76. However, it is still unclear what exactly this will entail.
Given the size of the game world, it will be interesting to see what Bethesda has planned for the long term. Quest lines and new regions have already been hinted at.
Instead, Bethesda seems to rely on microtransactions: Since Fallout 76 is designed as a game-as-a-service, the question remains how Bethesda plans to profit from the title. The answer lies in microtransactions. However, these are expected to be limited to cosmetic items only.
Bethesda promises that the microtransactions will not affect gameplay.

With a bit of luck, we will receive more information at QuakeCon. This event runs from August 9 to 12.
What is certain is that a beta is set to start in October. However, there were a number of misunderstandings surrounding this testing phase, which Bethesda has since clarified. You can read the details here: