https://cactus.chat/, Cactus Comments is a federated comment system for the web, based on the Matrix protocol. Good idea but I am not sure why this needs an appservice for synapse. I think it could also be just done using a single room and storing the post metadata inside the chat message JSON.
@davidbisset-phpc-social Probably a fine line but Contact Form 7 promotes integrations for which they get commission. Like https://contactform7.com/sendinblue-integration/