The “greenhill formula” (and its fine tuned offspring) using the bullet diameter, length, specific gravity, and velocity ... provides the basis for calculating the optimum rotation required for stabilization. Although rotating a bullet faster than the “optimum” has no theoretical deleterious effect itself, bullet construction that is not “perfect” ... say in weight distribution, uniform jacket thickness, and a host of other factors... will amplify the imperfection and may result in poor performance. Consequently, the best advice is to choose bullet/characteristics that fit in the range of optimum stability provided by a given twist.
So, to address the issue posed in the OP, it is evident that maqnufacturers make slow twist barrels for specific (shorter) bullets, and faster twists for their longer cousins. Yes, we still need slow twist barrels because over rotating a bullet with deficiencies, is unlikely to give the best results.
It is quite possible that a well constructed bullet may shoot “decent”, which does not rise to the standard of shooting “well”.
If manufactures thought their barrels would shoot every length of bullet well with a fast twist ie: 1:7 ...they would just make them all that way and not bother with extra retooling costs.
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Old Guys Rule
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