There was already some discussion when adding the disclaimer to NS7, the points are still true, see Nethserver-rspamd a new module - #32 by stephdl and follow-ups
- DKIM signing is required nowadays else your mails will be classified as spam
- altermime was used in NS7 to add the disclaimer and it’s even older now and still not maintained
Did you already try to add a disclaimer in rspamd by using the lua script as explained here? Maybe it’s an option?