I imagine the low engagement is because everyone who uses it knows it’s a bot and treats it as read-only

I don't think this is the case, others like [email protected] and @[email protected] get engagement, and the latter doesn't even post memes which most lemmings engage with with little regard to community or user.

Is it actually causing any problems?

See my other comment, but nothing technical.

It’s pretty trivial to ignore/block it if you don’t want to see it. If it’s actually increasing our server bill or anything like that then it’s not worth keeping, but if it’s not causing any harm then why bother defederating?

It is easy to block, but new lemmings aren't going to know all the platform features.

I'm not adamant about defeding though, if people are using it I'll keep it around. There are other solutions to the issue of new users going to All, seeing a bunch of Reddit reports with no engagement and thinking the platform is dead. Like having new accounts block the bot by default.