Valve says video game developers ‘shouldn’t offer a Season Pass’ if they aren’t ready to clearly communicate what’s included in the DLC.

You shouldn’t pre-order games, but people do it all the time anyway. Increasingly, people have gotten in the habit of pre-ordering DLC too, paying up front for content that won’t be delivered for months or even years,

A new section added to Steam's guidelines focuses entirely on season passes and includes new rules on release schedules for included DLC.