[Disclaimer: I am an Antidote maintainer]

Some CRDTs support garbage collection directly - if you run them in a causally consistent environment. Antidote is causally consistent and has a Set and Map implementation that work like this; for these CRDTs you don't need a global sync.

Thanks for the reply. Don't you negate some of the advantages of CRDTs by mandating casually consistent environments? Can you speak to that more?

This was true of the early CRDT designs, but they have improved since then. You will find plenty of inspiration here: http://dblp.uni-trier.de/pers/hd/b/Baquero:Carlos