The Horus Heresy was a one paragraph blurb about a general of the Imperium, Horus, rebeling against the Emperor. In the olden days of 40k (I.e Rogue Trader), the Space Marines were conscripts in power armour (sound familiar?), the primarchs were human generals, and the Warp was filled with vampires. The HH was a time of myth and legend, and as editions were being pumped over two decades, the more it was revealed. If you wondered why the Leman Russ tank was named after the Primarch of the Space Wolves, that was because back then, he was a tank commander.
Then 2005 came around and the first novel of 30k, Horus Rising, came out and showed no, the Imperium has always been kinda of a dickhead. Yes, the Emperor today suffers eternally on the Golden Throne not just because he's fighting daemons for 10k years, but also partly because he knows his empire has turned from an authoritarian secular regime with advance knowledge to an authorian theocractic regime lacking said knowledge.
In short, since 2005.