I never liked the "Klingons are different species" stuff from the novels (then again, I don't like the Novelverse in general) and it has already been contradicted in canon, even without the Augment Virus: two Klingons who showed up in TOS, show up in DS9, as 90s-style Klingons.
To be clear, the novels that established Klingons as encompassing multiple species were ones published in the early-to-mid 1980s. Many of these novels had a shared continuity largely built around the old Space Flight Chronology reference book and Diane Duane's
Rihannsu novels and John M. Ford's
The Final Reflection. When TNG contradicted much of the backstory of that 1980s novel continuity (which has no name), the novels mostly stopped referencing that continuity and were consistent with TNG and its spinoffs. Those novels were therefore published before DS9 even premiered on TV, and long before DS9's "Blood Oath" featured Kor, Koloth, and Kang in TNG-style makeup.
The term
novelverse is often used to refer to the continuity shared by many of the novels starting from about 1999-2000, with the 2000 publication of the first post-series finale DS9 novels often being considered its start and the 1999 standalone novel
A Stitch in Time by Andrew J. Robinson being "grandfathered" in. After the post-finale DS9 novels established themselves, new TNG, VOY, DS9, and ENT novels were all published that took place after their respective shows ended (and regularly crossing over with one-another and with original novel series like
Corps of Engineers and
Titan).
In this continuity, which again has no formal name, Klingons are
not multiple species -- they are one species, and the reason for their difference of forehead is consistent with ENT Season 4. The only innovation they introduced is giving Klingon language names for TOS-style smooth-headed Klingons and TNG-style bumpy forehead Klingons:
QuchHa' (TOS style) and
HemQuch (TNG style).
The 1999-2020 era novelverse is basically being retired, since PIC has contradicted and thereby nullified many of its events. The upcoming
Coda trilogy is the "grand finale" of that particular shared continuity, and going forward future Trek novels will, as always, remain consistent with canon.