the structure is quite different from reddit and cannot be "compromised" the same way reddit can.
on reddit, all subreddits are subject to the single centralized authority of Reddit Inc. If Reddit Inc. doesn't like your community, they can and will get rid of it. that simple.
On Lemmy, every instance / server is independent of all other servers. you are still subject to the authority of the instance administrators, and just like on Reddit, if you are on an instance and for some reason those admins don't like you, whether because you are breaking the rules or for some other reason, they too reserve the ability to get rid of you from their platform.
but, the beauty of the fediverse is that anyone is able to start their own instance if they wanted to. if you have a specific topic that you want to have a community for in the fediverse, but some instances don't want your community on their instance, you can always find a new instance or start your own. nobody can stop you from starting your own instance.
Does this mean that lemmy could be used as a durable repository for DD? Or are servers/instances subject to wipes for reasons out of the control of the communities? It sounds like the vulnerability is still there, just in a different and less corporate form. Like a BBS in a lot of ways.
there are flaws with any platform and lemmy has numerous.
the administrator of an instance could decide to shut down the instance, and everything hosted on that instance would become unavailable to everyone else.
so, i personally would not depend on any single lemmy instance to necessarily be an invincible repository for DD. anything can happen.
ape historian (apehistorian.com), aka u/elegant-remote6667 on Reddit, has been backing up reddit threads from superstonk and wallstreetbets and maybe a few other subreddits but i don't think theppshow subreddit. So, lots of stuff is backed up by this individual on his own equipment.
there are other solutions for long term record keeping, but everything will always be changing. i now have to go and find archive links for a bunch of ppshow subreddit links because i can no longer get them from reddit.
Alternatively, anyone is capable of manually archiving any web page using archive.org. DD authors that are extra diligent can make their DD in Lemmy (warning: unless i am mistaken, Lemmy posts have a character limit of 10,000 characters) and then take the link to that post and paste it into the box on archive.org to Save Page Now
archive.org likely contains the vast majority of posts from the pp show, however finding them would be the tricky part.