UART- a protocol or hardware?
Both of your first two links are, simply, wrong. A UART is a piece of hardware which can implement a number of different protocols which are used to frame asynchronous data streams. The U is an acronym for "Universal", and while it is effectively correct there is no reason a protocol could not be used which confounds the present population of UARTs - other than the fact that it's not worth the effort.

The different protocols handled use different numbers of bits for detecting start and stop conditions, presence or absence of a parity bit (and its polarity), and frame data lengths. Typically you can specify 5,6,7 or 8 data bits per frame. If someone were to insist that his/her data must be formatted into 4-bit frames, no existing UART chip would be able to handle it.

In part, this is a matter of definition. Merriam-Webster, for instance defines protocol (for this context) as


  a set of conventions governing the treatment and especially the
  formatting of data in an electronic communications system


Note that the hardware implementation is not part of the definition.