Not sure when exactly, but the AC'97 standard did not include any specs for any kind of synthesizer, only a codec.
So you could argue that in 1997, Intel already decided that a (MIDI-capable) synthesizer was not an essential part of an audio interface, with the underlying idea probably being that you could use software wavetable on the device anyway, if you wanted MIDI. But you would probably be more interested in either CD-ROM audio or mp3 or such.
So perhaps around 1997 there were some really low-budget (onboard) solutions that already dropped the OPL3. By the time XP came around, it was indeed very common to have onboard that was only AC'97. So somewhere between 1997-2002 was the transition I guess 😀