That's correct. You cannot take part in any Forsaken or Forsaken: Annual Pass content on a platform that you do not own it on. Shadowkeep does not include Forsaken or the Forsaken: Annual Pass, so if you choose not to purchase it on PC, any exotics that are tied to that content license or activities tied to it will not be available to you. You can purchase the Destiny 2 Forsaken Complete Collection on Blizzard and have that transfer to Steam; it is still available at the new price there on Blizzard, or you can purchase it later on Steam once Destiny 2 is released there. Or, you can choose not to purchase it at all and skip that content on PC. The choice is entirely yours as far as what you wish to access and spend money on. As far as if the activities and equipment are relevant; for the most part, old equipment will still be relevant as far as light level goes and their use in activities. They are still valid equipment. New stuff will come with Shadowkeep, though, so what you consider relevant or not is entirely up to opinion and your own thoughts.