Agreed, but let's not forget this is not a face to face classroom. These sentences have been programmed into a computer program. So, if by chance, or by intent, something was not included the program will mark it as incorrect. When I say by intent I mean that Duo could not possibly include every regional and colloquial version of the languages.
In this case of course, it is good English. Try reporting it tohelp out other learners. See here for some tips etc but most important the Guidelines:
I know. I wasn't asking for an explanation of why it's not accepted, just stating that it should be. The point of saying "X should be accepted" in a report is to make the developers aware that something was missed so it can be added, and in comments to make other users aware that someone has noticed and already reported it so they don't feel compelled to bog down the developer with duplicate requests. It wasn't an invitation for a lecture on how software works. I'm a software engineer.
I agree that people use those contractions and in most cases would be understood but we must determine when we should avoid using them. Examples:
Similar to "Today's Thursday" in your example, "Today’s meeting" is a possessive form in the genitive (possessive) case. It asserts that the meeting belongs to today and is not referring to some other meeting on a different day (and today is not any other day besides Thursday).
I am stating that it is a better practice to avoid anything that looks like a possessive in English if it is not.
Looking at this from the Italian perspective, I believe Duo is reinforcing that position by requiring the verb "is", written as "è" in this lesson rather than a contraction. I am not far along in my learning but I have never seen a contraction in Italian for isn't or don't for example. Elisions of the pronouns are common of course.