It depends on what you mean. I can't think of a standard markup for de-emphasis. You could imply it with color perhaps, e.g. some shade of grey, or size  by making it smaller. At least for LaTex/HTML, both of those can be done. You would either need to use something like a macro, or a link for something like that I think. I made a link for colorized text once (https://kitchingroup.cheme.cmu.edu/blog/2016/11/08/New-color-link-in-org-9-0-using-font-lock-to-color-the-text/) that you might be able to adapt.