Assuming a desert cactus could get all of its needs, light, heat, water... during the growing season met year round, does this cactus need a rest/dormancy period to remain healthy? Flowering does not matter, just do desert cacti need a dormancy period to remain fit and healthy. I am talking about all desert dwelling cactus as a group, not jungle or other speciality cacti. This is just a theoretical question.
It would seem not,though there is little research in regard to the health of any succulent plant should it not experience dormancy. There isn't much difference in the dormancy story between desert or any other type of cacti or succulent either; dormancy or partial dormancy can be triggered by periods of stress or drought, as well as the more usual cooler temperatures and reduced daylight hours. There are some cacti and succulents that will not flower without experiencing a degree of dormancy, but you're not asking about that anyway.
Succulents and cacti kept in a home environment may not go fully dormant anyway, since they're generally not exposed to very cold temperatures, water doesn't disappear completely, and daylight hours may be extended by certain lighting. It is,though, safer to assume that a desert cacti (or any other succulent plant) indoors may have a lower requirement for water during winter if it is not growing with the aid of growlights to substitute for daylight, when it might decide to become dormant or partially dormant.
Further, more detailed information here https://www.sublimesucculents.com/science-succulent-dormancy/
It certainly isn't "natural" for a desert cactus to have a continuous supply of water, but they seem to be healthy enough growing continuously for half the year if they do have sufficient light, heat, and water.
But in the UK I have no experience of trying to keep them growing continuously all year round with artificial light in winter, and for my collection water does "disappear completely" over winter - they don't get any water at all from about October to February. They seem to thrive, and flower, on that sort of annual cycle.