Don't any airlines T&C when booking advise that no particular make or type of aircraft is guaranteed .... I'm sure it's in there somewhere. It's the company that decide the plane ... route, distance, load factors of SLF and real freight, availability and staffing to name a few conditions.
If you want to be sure of the type of plane you're going to fly in, pay up for a business jet.
(Even the old BEA printed timetables of the 1960s/1970s used to state EQV = Equipment Varies!!)