In general there are still quite a few forge worlds retaining this knowledge.
If we're considering books as cannon material: In Titanicus don't they imply that they still produce some? If you have other cannon sources where it is stated (if they still know or not) and you remember it please name it.
ps I read a answer about the imperium having lost the knowledge of how they create Space Marines: Are there other characters enhanced in the same way as Hector Rex in Warhammer 40K?
Correct me if I am wrong but they still know, it is just that some chapters are better than others in the process or are more like mad mystical surgeons;
What the Imperium is unable to do is to perform modifications on the genseeds and create a whole new "Legion"(not the best word)