Genève and many Swiss towns does this. Bus tickets are from a machine (sometimes in the bus, otherwise at the stop) or SMS or app. Most locals have an yearly pass.

Fare control is done by six or seven controllers getting on the bus at once. Check is very quick. In the night they will be joined by a security guard.

Boarding is very fast because of it. Also makes it easy to get in with a baby carriage or wheelchair (relatively).

Toronto now has open boarding (i.e. you can enter at any door) through all its streetcar lines and many bus lines. The new streetcars have totally separated the drivers from the passengers, so payment is more of an honour system + fare cops* proposition, compared to the old "show the pass/pay the driver" paradigm. Boarding speed and fare evasion have both increased.

*: "Revenue Protection Officers", a totally non-Orwellian job title.

What do you mean by through multiple doors? Pretty sure that all buses are boarded through multiple doors.

orange line in LA, built in 2005.