Its clearly not a priority at this point, and to be really blunt it might not even be something they care to implement at this point.
Fatshark has had prior features that were promised but never saw the light of day. The most notable one is still probably the vermintide 2 debacle around dedicated servers that were eventually after like 2 years confirmed not to be coming.
I hate to say it but its probably just better to refund and check back in some other time in year or two if the feature has been finally implemented, rather than to keep the game in library dusting if you got no intention to play the game without the solo option.