Japan is an island nation, so how much fish was the average person eating during the ban? Was fish affordable for peasants?
The ban was based on Buddhist doctrine. Was there a reason fish were categorized as different than land animals?
Did the nobility not hunt in Japan or keep livestock like European nobility?
How were transgressors against the ban punished?
Was the emperor secretly sneaking steaks?
The 675 edict (by Emperor Tenmu) didn't ban meat-eating - it restricted meat-eating. It forbade eating cattle, horses, dogs, chickens (or just roosters?) and monkeys from April to September. The only wild animal on this list was monkeys, and other wild game could be eaten year-round.
It's possible that this had relatively little impact on meat-eating, since most meat that was eaten was probably wild game, and the only wild game affected was monkeys (and only April to September). In particular, wild birds, boars and deer (i.e., the most popular game animals) were unaffected.
Similar bans were issued repeatedly over the next century, suggesting that many people ignored such bans.
Bans such as these, and social disapproval, and religious ideas about pollution due to eating meat did cut down on the eating of beef and horse. Pollution required purification, and shrines required meat-eaters to purify themselves by prayer and/or fasting before visiting the shrines, with the time required for purification varying from 3-5 days for most wild game through to 100-150 days for beef and horse. Much of the meat industry was linked to pollution too, with butchering (and leather-making) restricted to the "untouchable" classes.
However, recipes for cooking meat continued to appear in cookbooks, and archaeology shows that animals continued to be killed and butchers (and therefore presumably eaten). Shops selling wild game meat did business in towns and cities. Beef was also available in a socially-acceptable form: medicine. Want to eat beef without incurring a 100-day obligation for ritual purification? Get a doctor to prescribe beef, and eat as you wish!
For more on these bans, and references, see my past answer in
That said, most people in Japan ate little meat. I haven't seen good statistics for Japan, but the Japanese diet had much in common with the Korean diet. Korean food included more garlic, more pork, and after the Columbian Exchange, more chillies, but much of the cuisine was similar. Meals were built around grain - rice for the wealthy, and barley/millet and/or mixed grains for the poor - often in the form of grain, soup, and three side-dishes. For the poor, one of the side dishes would often be fish (perhaps closer to weekly rather than daily) and very rarely bird or mammal meat. For the rich, meat and/or fish dishes would often appear on the table, often as multiple side-dishes (the wealthy would often have more than 3 side-dishes, but sumptuary laws limited the number). This description works for both pre-modern Korea and pre-modern Japan. Even coming into the modern era, Korean meat consumption was limited by poverty - even in 1960, Korean meat consumption (bird and mammal meat) was only about 5kg per capita per year.
In modern Japan, when we have reliable records, seafood consumption dwarfed bird and mammal consumption. In 1900, annual seafood consumption was about 36kg per capita (live weight, so about 20kg meaty bits). Meat production was about 800g per capita (carcass weight); this was mostly beef as pork production was only about 30g per capita, and chicken production even lower. Egg production was about one egg per month per person. This represents an increase in meat-eating from a minimum in perhaps the early Edo Period. Compared with Korean meat consumption in 1960 (by which time, Japan meat consumption had grown enormously), Japan's consumption in 1900 is similarly seafood heavy, and has even less bird and mammal meat.
Two things would have reduced Japan's meat consumption from about AD1000 through to the Edo Period: (a) meat-eating bans and taboos, and (b) the combination of the dependence on wild game as the main source of meat and population growth would have reduced the meat available per capita. From the above Japanese and Korean consumption data, we could estimate that the impact of (a) and (b) was a reduction in the average annual meat consumption per capita from perhaps about 5kg at the most to about 1kg.
The upper classes, including the nobility, had much more access to meat, including a wide variety of game. They also had access to "forbidden" meat without penalty as medicine.
For more on Japanese diet, see my past answer in
and references there, and also
Was there a reason fish were categorized as different than land animals?
While Buddhism treats killing as killing, whether of fish, fowl, or mammal, people do tend to treat them differently. Sometimes, this is justified by a belief that fish (and other seafood) suffers less when killed - we boil lobsters alive much more often than we boil pigs alive. Many people who abstain from bird and mammal meat but eat seafood describe themselves as vegetarian, despite the non-vegetable nature of fish. (Further, in some SE Asian cultures, chickens are sometimes described as "fence vegetables" to vegetarianise eating them.) There are also non-Buddhist religious reasons: Shinto disfavours the eating of horses and cattle (since these work for us) and roosters (since these also work for us, as alarm clocks). Monkey-eating is often viewed as rather unsavoury due to the close resemblance of monkeys to humans. Thus, Emperor Tenmu's list of banned animals (cattle, horses, roosters, and monkeys) doesn't look particularly Buddhist, even if Buddhism was used as the official justification.