where can I find more information on HE's "we treat peers as customers" routing policy?
As for the goofy routing: if you were an IPv6 peer they'd announce you to their a large amount of their other peers (normally you don't announce peers to transit or peers - just to customers). This led to lots of sub-optimal routing scenarios and you'd have to ask them to knock it off and treat you like a real peer. So yeah cool you get free transit kinda, but the goofy routing isn't worth it for a lot of folks. This plus their long history of lack of filtering their transit customers and enabling lots of route hijacks really gave them a bad reputation. Their recent news about deploying RPKI is a bit of fake news: it's not real RPKI and doesn't address the issues with their customers.
Wouldn't you just have an inbound filter to only allow HE ASes? Or do you also want to reach other ASes that pay HE for upstream service, but are multi-homed so exist in their own AS?
Received: So for the most part people implement zero or just minimal filters on routes received from peers. They might drop a set of ASNs they consider large that would be indicative of a leak. Some may go the extra distance and even do IRR filtering. But for the most part people are fairly permissive in what they accept from peers
Advertised: Here's the catch. You announce routes to HE and they're propagating it to networks you don't anticipate. This pulls in traffic from other HE peers you weren't expecting. You don't really have much controls here. You can try to prepend but remember it's the other leaked peers of HE that will generally set a better local pref to HE (by virtue of being a peer) so your prepends won't do anything.