A couple of things to note here:
First, as stated earlier, the Horrors are the big metaplot secret. The top players in the background (and I mean, THE top players) know about them and are arranging things so that metahumanity will be prepared and might be able to fend them off in the future. They aren't laying betting odds on it though. There is big, if underground and subtle, support for the space program, through Ares, to get people out into space and space colonies and to find a way to allow the magically active to exist there without being torn apart by the mana warp of deep space. This is one of the big, long-term goals.
Will the players, or any other person not on the dragon's or IE's rolodex be privy to this? No, not unless the GM wants them to be. And given how bad everyone has been spinning this (including me), no GM should seriously contemplate introducing this with less than a very mature and experienced group of players capable of dealing with high level intrigue and empire building. Not everyone can do that (the only reason I could is because of my love of world building games like SimEarth, Civilization, and the like, but even I have limits), therefore not every gaming group can or should try it. Talk long and hard with your group about such things before diving in.
Second, in the 4th world, the previous Scourge everyone is referring to, the most powerful Horror didn't make an appearance. Known by many names: The Great Hunter, The Horror Worshiped as a Passion, The Hunter of Great Dragons, the Horror feared by even the most powerful great dragons and immortal elves is Verjigorm. This entity is referred to in dragon legends as the creator of the Horrors (and possibly the dragons, themselves), and seeks to corrupt everything about the world the dragons have settled upon. But, during the last mana cycle, it didn't make a showing. It is known that it captured many dragons and great dragons and put them in astral cocoons to convert them into a Horror/dragon hybrid capable of Re-Naming, and thus reshaping, the world around them.
Well, given that Verjigorm is immortal, the long view suits such an entity. Why release them after the peak of the cycle, when the majority of Horrors can't cross over to assist in the destruction/corruption of dragonkind and the world they live upon, when one can just wait until the next peak and saturate the world in corruption and death.
A bit of digging reveals that FASA (which is back in play after many years) will be creating three other "Earth" style games to add to the Earthdawn (4th world) timeline. The tetrology of games will be: Earthrise, Earthdawn, Earthdusk, Earthfall. The first will be the age of dragons and their rule (rumor has it a player will be able to play a dragon, yes, a Shadowrun/Earthdawn power level dragon, take that as you will). The second is the 4th age of heroes we keep referencing. The third, Earthdusk, takes place after the time of Shadowrun during and just after the mana peak. We lose. It's post-apocalyptic like the Deadlands: Hell on Earth game. The fourth will be Earthfall, and will be full-on Exalted level of badassery power level. Think troll Sky Raiders captaining space cruisers. The Horrors will be dealt with during this era, so other things are going on. We blow up the Earth so the Horrors no longer have a link to this universe. It's kindof a darker setting.
It has been said, however, that the link between SR and ED will be severed, and each allowed to explore their own way as an alternate timeline sort of thing. Which I think is the best way of doing it. Your GM can link them together or not, as each group wishes them to be.
In closing, you can use the Horror connection, or not, in your game. It's up to you and your group.