Are I/O buffers of ICs design to have 50ohm impedance?
Very few do. If your chip's datasheet does not say it has 50-ohm termination, then it almost certainly does not.

Traditional CMOS and TTL logic does not provide matching termination, though a few specialized types (line drivers?) might. Typically drivers are low impedance and receivers are high impedance (with some capacitance).

Traditional ECL (emitter-coupled logic) logic does not provide matching termination, though this family is often used at frequencies where (external) termination is desirable. Typically drivers are about 5 ohms, and receivers 2 or 3 kohms. Some newer parts designed for very high frequencies (above 2 Gb/s?) may provide on-chip termination for inputs (which may require an external connection to set a termination voltage). 

CML is the one logic type that I know of that typically does provide matched sources and receivers. 

LVDS I don't use enough to know what's typical.

For RF chips, consult the datasheet. Some will and others won't provide on-chip termination.

One advantage of not having on-chip termination is that this leaves the user free to use an alternative trace impedance like 75 or 85 ohms. It also allows multiple receivers to be connected to a single driver.