The reason Hashcash didn't take off with email is it imposes a marginal cost on email servers that is higher than the marginal benefit of email. Servers would either need (a) to purchase hardware similar to Bitcoin mining rigs or (b) pay someone else to do the hashing for them. The spammers would pay for that same hashing hardware/service. The servers and spammers would continuously rise until either the servers give up.

Spammers make an enormous amount of money sending ransomware and phishing emails. Email server operators make little if any money from providing email hosting.

I recommend reading about how Simplex Chat handles spam. In order to contact someone, you share QR codes with each other. This raises the marginal cost to send spam messages to each person far higher, because it requires that a spammer get that QR code or hack the device of each user to get that data. And if a user starts spamming you, you just delete that user from your contact list.

Nostr developers could adopt Simplex Chat's approach and eliminate spam within a few months, but I don't think that any developer will, because that is antithetical to social media. Nostr could be so much more tha social media [1].