1.We have some customers wants to know do we have any official doc to inform them that D365 FO does not support to use Excel add- in? 

From MS product team confirmed, MS didn't share the type of doc or TSG to inform clients.  and some external doc to asking AX2012 support to use excel add-in, but this feature is no longer available in D365 FO which is true.

2. Please share some docs for external user's awareness? thanks in advance.