but is dropping the preposition in accordance with ICAO standards?
No idea.
I tried to get a look at ICAO Doc 9432 Manual of Radiotelephony but they want to charge for the privilege. I therefore presume that their ICAOships don't particularly care whether we have access to standard RT or not.
FWIW, the UK CAP 413 Radiotelephony Manual gives the following examples:
descending to height 1000 feet
climbing to height 2000 feet
climbing to altitude 2500 feet
climbing FL280
descending FL90
They don't make it easy
