If it's been unlocked in your Collections, then you'll be able to use the ornaments at any time. Just like all other Seasonal content, though, it'll only be available through that season or through special engrams like the current Best of Year One Engram. So if an entirely new person starts up in Shadowkeep, they'll only have access to things they gain from that point; pretty much just the same thing that everyone else that didn't gain something during a season does.