If you're training each muscle twice per week it is less likely to happen because the time required to increase load enough to get significant DOMS at that frequency is not viable for most people, hence most people do not experience it long term. I say time because you quickly reach the limit imposed by weight and must incorporate extra volume to increase the load. There is also increased risk of 'plateauing' as it is not incremental overload and is very taxing on the body for natural trainees.
If you are doing workout X for a muscle group, twice per week and seeing regular incremental increases in the weight being used, most would agree all is well. One day you complete this workout, then do 10 x 10 of an exercise to finish that muscle group because, well, you feel like it. The next day you will be sore because you have introduced an extra-ordinary load. Regardless of the prior frequency of training.
I do not disagree that progression, whether it be strength/training density/physical size, generally a combination of the above over time, is the only measure of training efficacy. That is exactly why, regardless of whether you are getting regular or zero DOMS, these factors are what determines whether you are training 'properly', not the level of DOMS you experience.