Update:

G.R.R. Martin denies these rumors being true:

http://georgerrmartin.com/notablog/2019/05/13/idiocy-on-the-internet/

Thanks to haxxily for the update.

This rumor comes from the mouth of actor Ian McElhinney, who played Barristan Selmy on Game of Thrones:

31:39

https://youtu.be/FeftoI3c81c?t=1899

“George has already written Books 6 and 7, and as far as he’s concerned there only are seven books. But he struck an agreement with David and Dan, the showrunners on the series, that he would not publish the final two books until the series has completed. So if all goes well, in another month or two we might get Books 6 and 7, and I’m intrigued to know how Barristan, for instance, ends up going through those final two books. George, I talked to him during Season 1 and he did say to me that Barristan had a very interesting journey. But unfortunately I didn’t get to play all of that, so we’ll have to wait and see.”

Reactions?

Outside of a few bits here and there, I have never watched the show. From what I've heard though, especially the last few weeks, the show has become in the eyes of some fans, anywhere from "just bad" to "total abomination", and this has been happening to some degree since the show ran out off book material to draw upon. As for a reaction, that this kind of agreement would be struck makes sense. Both from a logistical and $$$ standpoint.

If this is true, then a lot of fans will be angry .

If this is true they would’ve still had material to go on. Unless the unpublished books are as illogical as the last few episodes of the series has sadly become. So I doubt there’s a fully written final two books on the shelf in GRRM’s bedroom. However I would believe that there do already exist outlines that could be more or less detailed. For example I think it was said that the ‘Hold the door’ moment plays out similar in the unpublished books. I might remember that wrong though but as an example.

From what I hear, the main problem with the later seasons of GoT seems to be that they ran out of source material.
At least that's what the decline in quality and nonsensical inconsistent character development is being blamed on.
If it turns out that that was a self-imposed ego trip on the show writers part, then it's just really really the fundamentally stupidest route they could have gone.

Doesn't make much sense, as GRR could have released book 6 by the time that season ended and book 7 in 2020... no reason to hold both.

Writers of the show probably didn't want to have any attention taken away from the show.

If this is true at least fans of the books can have hope to see the whole story being told and finished.

I'm gonna say this scenario is unlikely. But if true, any shred of what little integrity Martin had left is out the window.

Well they didnt run out of book material though, they deliberately branched off after book 3. If I recall, at one point GRRM stated that he thought it would take 3 seasons to cover books 4+5 but they managed to streamline them both into a single season and that was when things really went downhill.