Most standard urban benches do come with armrests, and this is by design—not by accident. City planners and furniture manufacturers install armrests specifically to prevent people from lying down on them, making the benches inhospitable for overnight sleeping. This is a common strategy in urban design, sometimes referred to as "hostile architecture.
However, the answer changes depending on where you look. In some parks, recreational areas, or train stations, you can find benches without armrests—these are usually longer, flat surfaces designed for seating flexibility rather than anti-sleeping measures. If you are buying benches for a private property or a commercial space, you absolutely can order them without armrests. Many manufacturers offer "armless bench" options as a standard variant, especially for outdoor settings like hotel pools, resorts, or private gardens where relaxation and flat resting surfaces are encouraged.
If you already have benches with armrests installed, modifying them is usually not possible without damaging the structure. Some armrests are bolted on, but loosening them may leave sharp edges or holes. For easier sleeping access, your best bet is either to purchase no-armrest benches or look for "integrated sleeper benches" that are designed with flat surfaces in mind.
In short: armrests on urban benches are the norm, but you can go armless if you're buying new, especially from vendors that prioritize comfort over anti-homeless design.