I think there is zero chance of either of them blocking Twilio (which is what Nextdoor uses), as their users would riot. Twilio aren't going to drop a customer who they prominently feature[0] for sending emails to customers to whom they have an existing relationship.
The only kind of spam anyone really seems to care about is truly unsolicited email. A publicly listed company with an aggressive stance towards emailing their existing users, nobody is going to get excited about.
All I can say is IPs can and do get blocked by email receiving services, and especially Gmail and MS with their automated feedback loops, with all the implications of collateral damage that it entails. I remember helping provide data points in a similar conversation with another large customer (and a brand name that most would recognize here) who was concerned about increasing complaint rates.
The problem, if anything is that people kinda don’t know about what to do with these emails, and if more people marked these emails as spam, senders such as Nextdoor would sit up and take notice.
The only kind of spam anyone really seems to care about is truly unsolicited email. A publicly listed company with an aggressive stance towards emailing their existing users, nobody is going to get excited about.
0: https://customers.twilio.com/en-us/nextdoor-2