What is the current state of the Hashcash protocol that is cited in the Bitcoin whitepaper?
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.