Is this sentence true or false?
I deem it an honour to accept your invitation.
If it's right, why not is " I deem it is an honour to accept your invitation. "

Question

Is this sentence true or false?
I deem it an honour to accept your invitation. 
If it's right, why not is " I deem it is an honour to accept your invitation. "

Answer

That is very old fashioned. It  is an honour to accept your invitation. , is more natural.

Answer

A correct rewrite is ''I deem it to be an honour...'', even though the rewrite is wordy.

Your use of indicative ''is'' really doesn't fit here.  You're talking about your opinion.  Understand ''deem'' as similar to ''consider'' (but more decisive).  ''I consider it an honour...'' is also correct.