At this time, the only way to acquire access to Season 20, and by extension the Vex Excalibur exotic glaive, is to purchase the Lightfall Annual Pass expansion, or the Lightfall Annual Pass upgrade if you own Lightfall already. Some descriptions for Lightfall incorrectly detail that season 20 is included. This is likely a carryover from when Lightfall was originally launched during Season 20. The correct description is that the current season will be unlocked upon purchase. Looking at Steam's storefront, the description for Lightfall correctly details the current season being included. If you can reference where you are seeing Season 20 listed as being included in Lightfall, I can pass that onto Bungie for review.