As far as I can get it (see my location
) the most commonly used phrases are
"climb/descend Flightlevel 230"
"climb/descend Altitude 5000ft"
No "maintain" and no "to" ... short and sweet
... even so (nearly) no chance for any misinterpretation ... (???)
My assumption is that the "history" of the phrase "climb/descend xxxx and maintain" has something to do with step climbs/descents associated with wrongly adopted continous climbs/descents ... ( ? )
I also assumed that there were some "incidents" caused by such misinterpreted climb/descent instructions ... like the phrase "ready for take off" was amended to "ready for departure".
... or something like that.
regards,
Redfox