It's the normal way of saying it. The members of the Swedish academy pronounce it this way when they read their speeches aloud, so there's nothing informal about it. However sometimes you can hear people say de when they read aloud because they are influenced by how the written language looks. This happens more often to people who don't read aloud a lot.
Same question out here. I´ve been wondering the same thing. I've also have a memrise account and they say it the same way, I´ve watch youtube videos and they say it like that.
I guess it is post of the exception of their language, like every language have their own exception. :)
Some words have to delete last vowel before add "ar" or "or", some just put "r" or "er" at the end, some use "n" or nothing changes in plural form.
More details https://sv.wikipedia.org/wiki/Svenk_grammatik#Numerus
It’s normally pronounced as just a plain ”e” or ”ä” depending on dialect, therefore if the previous word ends on that sound they often form just one longer vowel. So ”det är” is normally just ”de e” → ”dee”. ”De” as in ”they” is normally pronounced ”dom” but I think the computer voice pronounces it as it’s written, and then it’s possible they merge in the same way.