Is owning the expansion a requisite in order for newer exotics to drop from legendary lost sectors?
Over the past two hours I've just run today's lost sector about seven or eight times in a row. In that time frame I've seen two exotics drop, both being the same exact exotic and neither one being from the current season.
Having run other legendary lost sectors numerous times and the only exotics that have dropped are ones that already exist in my collection, I'm wondering whether the newer armor pieces are locked behind a paywall (similarly to most of the new exotic weapons)?
Correct. In order for the exotic to drop you need the correct expansion/season. For example, the 6 new exotic armor pieces require the Witch Queen DLC to obtain from Lost Sectors. When the season changes to whatever follows Season of the Risen, you will need that Season to obtain any new armor exotics from Lost Sectors. Once the Destiny year changes (with the next expansion) you will only need the Witch Queen DLC for these exotics as the season passes will be removed. An example of this would be any exotic armor from Season of the Lost now just requires Beyond Light to obtain, as Season of the Lost is no longer a part of the game.