To “86” means to get rid of. Normally that is very ambiguous and non threatening.

Trump admin and supporters believe a recent social media post referencing 8647 is meant to encourage murdering the President.

Unknowingly, perhaps, indicating that the only way to get rid of Trump is to assassinate him implies all other legal methods are no longer valid (they aren’t but they like to pretend they are all about law and order and value the legal system).