The technology levels are generally speaking, limited to no later than than the 15th century. Gunpowder, electricity, and combustion engines simply aren't, as is mass industrialization. (Once you lose the infrastructure to mass produce, it can be hard to bring it back.) But some things aren't so much technological advancements, as refinement of low tech. And some things just seem like a neat idea, but have been confined to things like toys or coats of arms. Even the umbrella or monocole is occasionally used. Here are some later technologies that remain as symbols or flights of fancy. Not all--nobody has really tried to do anything with the cotton gin.
Any train tracks or train stations have long been gobbled up, but they lasted much longer than asphalt roads, which made them much easier to travel on for a generation or two. The train also featured heavily in the mythology of the western, and various Christmas stories, and of vintage America in general. As a result
Technically, people can make bicycles. But it does require so much precision materials as to make it exorbitantly expensive, and roads, as well as most medieval fashions, don't make it very useful to ride. But if one looks at medieval toys, it was very common to see toy knights attached to horses, in turn, attached to wheels. Toymakers sometimes cut out the middleman and depict figures riding a bike, simply because the design of being upright and rolling works too well.
Cannons could be found in plenty of parks and memorial sites, and generally stood as points of pride for various towns, so they tended be melted for scrap much later than automobiles and the like. As a result, cannons retained something of a powerful symbolism, and it shown up as a heraldic charge or in a few tapestries. (To the the point old-fashioned cannons shows up depictions of things like Pearl Harbor)
Bullets stopped being a thing, but squirt guns, nerf guns, and most commonly pop guns still works, and so rifles still live on in toy form. Otherwise, they are somewhat forgotten, and when statues or pictures of people holding guns show up, it's assumed they're holding pikes where the end came off. Thus, restoration projects tend to "fix it", and turn their firearms into polearms.
Fun fact, one can technically make an approximation of a telephone using a string and two cups. Nobody ever really has use of it, but once again, it shows up as a toy or piece of craftsmanship to encrust jewels or carvings into, with superfluous dials, and the ability to use telephones is one of the symbols of the lost Golden Age of civilization.
Traffic lights are sometimes approximated--making lanterns of red or green tinted glass, usually at places where people have to pay a toll, or to open or close markets.