If there is a one source that could have beet kept accountable for propagating this myth, it is likely to be The Englishman's Food: Five Centuries of English Diet written by Jack Cecil Drummond and first published in 1939.

One can be tempted to attribute the fallacies of a discussed claim to the fact that Drummond was not a historian, but it should be noted that he graduated with a degree in chemistry and was the first professor of biochemistry at the University of London, holding that position during the work on his book. It is thus pretty strange that such claim came from a person who should have been very familiar with the real influence of spices on the products of putrefaction and the influence of the latter on human health.

Drummod says specifically that: 'It is not surprising to find that the recipe books of these times give numerous suggestions for making tainted meat edible [...] The popularity of strong seasoning for meat was undoubtedly due to the frequency with which it was necessary to mask taint.' [op. cit., pp. 36-37]. It is worth pointing out, however, that by 'spices' he means both exotic spices and local or at least adopted seasonings, such as European herbs, garlic and onions (the latter, although now ubiquitous in European cuisine, are relatively young addition, having been popularized only in the Crusades era).

It is also very possible that the idea stems from the general condescending view of the Middle Ages, as a regressive and unsophisticated period that was quite prevalent in modern historiography and now is equally well-entrenched in popular consciousness, at least in the Euro-Atlantic area. I made this conjecture because whenever the myth of using spices to mask the smell or taste of spoiled meat, only the Middle Ages (that are, by definition, a period limited to European historiography) are invoked, with other periods or cultures virtually never being mentioned despite substantial number of mentions of meat consumption or usage of spices. The latter were often mentioned in relation to Arabic, Persian or Turkic cuisine (and in the Age of Exploration also to one on Indians and Native Americans), but for all these peoples are always depicted using spices to season their food, never to mask its smell. For some unfathomable reason, what is simple a cultural preference or exploitation of opportunity for everywhere else, in case of medieval Europe becomes a means of damage control. It is even more perplexing when one realizes that a cuisines closely associated with substantial amounts of fragrant spices, i.e. that of Indian subcontinent, are largely vegetarian, at least partially because of religious reasons.

It is worth noting that people might have tended to shun the meat that even looked suspiciously, regardless of their actual smell or taste, as noted in Menagier de Paris, a housekeeping handbook published in 1393, where it is said that 'some hang their pigs in the Easter season and the air yellows them; and it would be better for them to keep them in salt as they do in Picardy [...] because however good the yellow may be, it is too repulsive and causes disgust when viewed.'

The prevalence of the myth might be also fueled by the relative lack of first-hand experiences of people in the 20th century who, thanks to the extensive supply chains and technological advances, such as canning and refrigeration, rarely had opportunity to find a spoiled meat and thus were not aware of a serious ramifications of eating such products. Humans are natural bivores and thus are incapable of eating spoiled meat, and although slightly unpleasant smell might only be a result of the presence of Lactobacilli bacteria (the same microorganisms responsible for fermenting milk and hailed by dairy producers as 'boosting immunological system') at which point meat is still edible, the results of putrefaction process, namely substances like ptomaines or putrescine not only make the meat revolting to human smell, likely inducing gag reflex and vomiting, but when ingested, they might lead to serious poisoning that until recently was likely to be fatal.

Please note that high effort to raise animals meant that meat supply was relatively scarce. Not necessarily expensive, as until late modern era large part of most societies were composed of prosuments of comestibles, i.e. producers who also consumed part of their own products, and thus were self-sustaining in relation to the products they manufactured. But meat was scarce, what meant that it supplemented daily diet and thus in case of spoilage, people did not have to risk their health or even life, as there were many other sources of nutrients readily available.

Thus, the idea of masking spoiled meat with spices seems to be relatively absent prior to 20th century and it can be traced back to the Drummond's book, not unlike the common idea that crossbows were specifically outlawed by Church dating back to The Crossbow by Ralph Frankland-Payne-Gallwey, published in 1903.