I cannot say that it isn't recommended, because recommendations are quite diverse. But I haven't seen that recommendation before.

Orthodox nutritionists would definitely advice against ketosis, potentially they may be fine with fasting-induced ketosis every once in a while (depends a bit on how they were trained).

People who embrace a low-carb ketosis-inducing diet (aka "Keto diet") or embrace multi-day fasting don't consider it harmful really. For fasting there are of course certain limits but not due to ketosis but for other effects of not eating (refeed effect, general weight-considerations, etc. but there have been overweight people successfully fasting for months under supervision).