Technically it is still there. However, when a user is banned, you can also choose to remove their content. You could choose not to, but then what's the point in automatically banning a spam account if you have to go and remove the spam posts yourself.

If you choose to remove them all, and you accidentally hit a real user, you'll remove all their posts and comments. Lemmy doesn't provide an easy way to restore the content. And although there are automated solutions, you come to the next problem of knowing which posts to restore. Many posts were removed by mods of communities, many were removed by the user themselves. You don't want to restore those items, instead you need to remember which you removed and restore only those ones - this is different functionality to Lemmy's option to remove all their content.

This actually exists in some form, there is an AutoMod that keeps a log of removed content for banned users and allows a restore of that content. So it's a solved problem, just would need a similar solution to be built for a ban list.

One thing you'll learn quickly is that Lemmy is version 0 for a reason.