As I have learned last week, you don’t really own your data on Mastodon. It can just disappear overnight. And even if you made a daily backup, that would be incomplete, and you cannot import all of it.
The main reason for this deficiency is identity. My identity changed a couple of times from @email@example.com to @firstname.lastname@example.org to @email@example.com to @firstname.lastname@example.org. Many people will never change their identity, but when they do they leave something behind.
Everything you post, contains the name of your server (in Fediverse parlance: instance). Example: https://social.heise.de/@vowe/109763782893905881 If that server goes out of business, all links to this post will 404. You can backup the post, but you don’t own the URL. Should the server just disappear or ban you, it’s gone.
Some of your data is portable when you move to a different server. You can see those CSV files: Follows, Lists, Blocks, Mutes, Domain Blocks and Bookmarks. If you back them up in time, you will be able to upload them to a new home.
Since identity is tied to a server, it needs to be updated when you move from one server to another. Mastodon does that for you. You can tell the two servers to move your followers. This looks like magic. Your old accounts get marked as moved and then your followers will change their follow list, completely automatic.
This magic is also dangerous, because it alters data en masse, data that you do not own. Example: I blocked one instance, as per my export. When you do this, your server tells the other server to remove you from the follow list of all people on the other server. There is no undo.
As I have shown you last week, it is quite easy to strike an instance because you select this nuclear option on an innocent looking menu on a single person entry. Once you do this, you will lose a part of your data with no backup.
You learn from your mistakes. I have learned my data is not safe in Mastodon. Last week I lost around 900 followers on mastodon.social which is the largest instance on Mastodon. I have exactly one blocked person on this server:
I cannot really remember why I blocked this account, but the handle suggests it was one of the spammers that have plagued this server. I don’t really understand why I have been removed from all my followers on mastodon.social, but a few of them have found I was missing and re-followed me although I am currently taking a break from Mastodon.
You can architect identity in a different way. did:plc:k5ybhnnyl4zwq7ey3q3fogzt is my identity in atproto, the protocol that drives Bluesky and soon many other services. I use my domain @vowe.net to resolve to this identity. That is my name within Bluesky. I can change my name but it will still resolve to this identity.