There is no serious usage, and has never been, of Hashcash.

Most obviously, the disparity between the resources of a spammer (who can rent a botnet of stolen compute resources, or use high powered servers and GPUs) is no match for a user sending an email from their phone with limited power available . This simply means that the difficulty of generating a "stamp" has to be so high that only spammers are able to send messages, which defeats the purpose.