Did the age of gunpowder completely destroy them all, lack of need for them make them redundant, or lack of maintenance reduce them into nothing? Did they slowly get destroyed to use as building material? Or do some still stand just not as high with the development of cities?
Me, u/RenaissanceSnowblizz (for the benefit of automod not bugging me about self-referencing)
wrote a bit about various things that has happened to city walls in various places. Unsurprisingly it depends on locale. Seem are gone, some are preserved, some were long replaced by new types of defences. Basically yes all of the above.