The spammers are taking advantage of open fediverse instances - and it's not one, but instances chosen at random. You may think that banning instances would solve it, but it will block other legit users.
Some of the automod bots are also filtering and blocking posts based on the content as well (RegEx, confidence rating based on the age of the account and upvote/downvote ratio, presence of rarely seen domains in URLs), so that helps a bit.
Most of these are unfortunately limited to local instance moderation, which means all instances that don't run these bots don't benefit from them.
The posts have already been modified in ways that they aren't as easy to reliably filter anymore, though still possible with fairly low false positive rate.
To add to this, depending on how content is removed, removal may or may not federate properly. on Lemmy.World, we've been removing content in a way that reliably federates, so while a lot of this spam does arrive in !asklemmy@lemmy.world and !selfhosted@lemmy.world, the removals on Lemmy.World should federate to all other instances (0.18.5+).
It's weird.. I haven't seen a single spam post here. Only posts about the spam posts. I don't think I have any filtering on my end. Not that I'm complaining, but why don't I see them?
Because your instance admins bulk deleted the posts directly in the database, which doesn't federate to other instances.
Quick and dirty fix because Lemmy is currently ill equipped to delete mass spam originating from many different accounts, but it means people from other instances still see all the spam and the lemmy.ml based moderators can't do much because it's already gone on their side.
The spam I noticed was yesterday, might have been removed by the time you came online, since they where identical and likely easy for your admins to filter out.
I'm happy to help moderate. I currently mod several small communities here on lemmy including !boinc@sopuli.xyz and !scientificcomputing@lemmy.ml and would gladly add this one. I also have experience modding some reddit subs.
That's not a problem specific to reddit. It happens because only a small portion of people step up to moderate. You'll see the same dynamic play out in all leadership roles whether you're talking about reddit, your local government, volunteer organizations, open source projects, etc.
You can be part of the solution if you step up to be a mod.