I’m sorry but we can’t implement mail disclaimer on the server side to avoid breaking digital mail signatures. As alternative configure the disclaimer in the mail client. If you use the Webtop groupware you can look at Mailcards: WebTop groupware — NS8 documentation.
Edit: if someone wants to try, Rspamd’s developer published a custom script that adds a footer at pre-filter stage: footer.lua · GitHub. Disclaimer - I don’t known if/how it works.