No, not yet, but the Japanese version has been made, so the English one ought to come out eventually, like Obelisk.
There is a game legal version, which was released in Shonen Jump magazine and has since been reprinted a few times. The Asian-English GBI version, and the English versions from DOD, and LC1, are not game-legal.
There are currently no TCG rulings for the Effect monster, "The Winged Dragon of Ra", because it hasn't been released in English yet.
The answer to this question cannot be determined with Slifer the Sky Dragon or The Winged Dragon of Ra because Slifer the Sky Dragon's legal copy has yet to be released and because The Winged Dragon of Ra's (legal version) effect allows a Player to pay Life Points until they only have 100 left to add to the ATK of The Winged Dragon of Ra equal to the amount of Life Points paid.Considering only ATK and DEF, Dragon Master Knight has more ATK and DEF than Obelisk the Tormentor in a sense that it is "stronger".
Winged Dragon of Ra's game-legal reprint differs from what most people expected, it is not the same as on the show. It does not gain any ATK from the monsters that were tributed for the summon, but when you summon it, you can pay your life down to 100 (not 1) and it will gain as much ATK as lifepoints you paid. (The card isn't available in English yet, but has recently come out in Japan and Korea, and like the legal version of Obelisk, the TCG should get it too within a few months)
For the legal Winged Dragon of Ra, you must pay all but 100 Life Points.
Japan gets theirs earlier than we do in the west. The legal version of Ra has already been released in Japan, so the English print should follow. Since they've done Obelisk and Ra, there's no real reason to think they won't do Slifer too.
Actually yes
no, but they are making one
There is currently no game-legal version of Slifer the Sky Dragon.
It's not 'banned', it's more accurate to say it has never had a legal printing. All 'English' versions have had "This card cannot be used in a duel", and all Japanese prints did too, until a legal reprint in V-Jump magazine.
so far, there is no combination of the god cards made. but, it could be a possibility in the future, once they release tournament legal slifer.
It was released in Shonen Jump magazine in December - Japan got their copy about a year previously.