I have tested this config option by using https://walshy.dev/blog/21_03_16-mc-new-text-filtering as a guide. The config option works on vanilla 1.17.1 and vanilla 1.18 pre5 but it fails to work properly on spigot 1.17.1 and spigot 1.18 pre 5. 

When the config option is configured with an API endpoint it should send 3 different api requests, one for player joining, one for player leaving and one for every chat message sent.

Currently on spigot only the player join/leave api request is sent while the chat request is not being sent and this is the reason for this bug report.