If I'm remembering correctly, my retainers kept all the items they had on them when I unsubbed for a break.

I've got a bunch of retainers and am still ruthless with inventory management. If it's not something I'm going to use this expansion, it's gone. I do keep one retainer dedicated for housing and another for glamour/extra gear for alt job leveling so they get full sometimes. All the others regularly have the majority of their inventory liquidated to keep space for thing I do need or will use.