Depends on type. LM2596 for example, I believe can run up to 100% duty cycle, but has a Sziklai type output stage, so drops about a diode drop minimum, or about 5.8V minimum input (unless the control needs more; and certainly more at higher load current, hence the 7V recommended minimum input).
Most bootstrap (NPN/NMOS) buck converters will not run up to 100%, as the bootstrap supply needs less than 100% to remain charged and active. The minimum pulse width (high/low) or maximum duty is usually given, which will in turn tell you the dropout.
Complementary types (PMOS switch) can run up to 100% with low dropout (Rds(on) limited), though they might not for other reasons; they're also less common, and seem to have less desirable controls (like voltage-mode or hysteretic control). YMMV.
You can certainly design one yourself, to behave that way; even using NMOS switch, using a charge pump or isolated supply to power it statically. This is usually more work than it's worth, though (or there are controllers specific to the application, like wired-OR or hot-swap controllers).