I will tell my clients that IRS might change its mind about this. The law allows IRS to extend the due date for filing returns, but not to allow additional time for other deadlines unless it comes under the category of “Any other act required or permitted under the internal revenue laws specified by the Secretary.” See Section 7508A, which refers to a list in Section 7508. Mnuchin has been busy with other duties lately, and I haven’t seen anything in writing from his office.
For most people, if they haven’t decided to save for retirement by now, they are probably clueless about many other financial matters. But for some, who want to contribute only the amount they can deduct, wanting more time is understandable. However, many have always had a need for such additional time, because they don’t yet know their income from K-1's, and until now it has just been tough luck.