I am on the verge of relaunching my fandom-centric forum that's been set to read-only since July. Since opening it the first time in 2006, I learned a lot about adminship and being a moderator too.
There were two separate incidences where a member wanted to leave the forum. Beforehand, I gave a member the chance the delete themselves by pressing a button and then confirming the choice.
The first incident had a member doing just that, but before doing so they went through their entire forum history and deleted everything. I was using SMF at the time so all their posts became 200+ new topics in a hidden recycle bin board. To my knowledge there was no 'restore' feature. The conversations this member participated in suffered as a result.
The second incident involved another member. They wanted to leave but knew the recycle bin existed. They threatened legal action if I did not remove every single post they created on the forum. I told them no, but I allowed them to delete images they created, such as photos. That also left holes in the forum threads.
I am unsure how to write a good licensing document that explains the difference between conversation and commentary with things such as artwork and fiction (or any very large body of written work). I want to make it clear that conversation cannot be deleted for the sake of the archive. This also brings in another problem where small drabbles and quick art sketches sit on the boundary between conversation and actual 'property' which may be contested to be removed if a member decides to leave or wants any part of their forum history removed for whatever reason.
(I am new to StackExchange, so apologies if I made any faux pas in the process of posting this.)