It was never much of a slogan in the first place. Before the last election, Conservative opinion researchers reported that floating voters were unsure what “levelling up” was supposed to mean. When it was explained to them, they interpreted it as an attempt to make their part of the country more like London, which they did not like at all.
On the other hand, it was vague enough to serve a purpose for the Brexit prime minister of a Brexit government. The point of leaving the European Union, ran Boris Johnson’s argument, was to give those parts of the country that had not shared in the prosperity of the international elite a fair chance.
It was poor economics, because EU membership enriched the whole country, but it was crudely effective politics in places where change seemed more of a threat than a promise.