They can still be used but unfortunately their power level is capped at 1100.
Bungie decided that it was time to retire some of the gear. The idea was for all items to have a set lifespan, they have since scrapped this idea but all items which had been sunset will remain as they now are.
Edit: typo
Pretty much. They can still be used in regular PvP if you wish, but for everything else they will drop your power level. If you’re hurting for vault space then they should probably be first to go.
You can make ornaments of your favourite armour if you wish using the new transmog feature - Ada1 offers 10 bounties per class per season for doing so.
Edit: Typo