Backers from before Epic deal got Season Pass, yes, but it was available for sale on Epic before Year1Edition came out - it was also included in the most price version of PP available on Epic.
I don’t think there is anything similar available for Year1One, and DLCs just will be available for purchase as they come out.
There are indeed 5 DLCs planned, 3 of which were revealed and Festering Skies is the last one of those three. There are still two more to go, though it is unknown what they are, or even if devs decided on what they are already - one account of Festering Skies coming out soonish, it is something they should start planning or even designing for already.