Would we send 73s in CW?
There is a problem in with assumptions on bandwidth as not all driver radios have SSB filters able to meet a 2.7 kHz limit.
So why wasn't the limit set to be compatible with the "standard" 300Hz-3.4kHz ? It's not as if there is a lack of bandwidth available with the extension to 500kHz.
Rec. ITU-R BS.640-3 might have been more sensible.
"The upper limit of the audio-frequency bandwidth (-3 dB) of the transmitter shall not exceed 4.5 kHz with an attenuation slope of 35 dB/kHz and the lower limit shall be 150 Hz with lower frequencies attenuated at a rate of 6 dB/octave."
However that is for SSB broadcasting and even though QO-100 is a broadcasting satellite, amateurs don't broadcast.
ITU Report M2478 which although related to the 6m allocation, does mention the bandwidths required for narrow band modes (CW/SSB/Digimodes) to be between 500Hz and 3kHz depending on the mode. So for 6m at least maximum of 3kHz.
Then there is the handbook: https://www.itu.int/dms_pub/it…HDB-52-2014-OAS-PDF-E.pdf which is where I assume the 2.7 kHz comes from though the definition of SSB:
SSB – Amateur single sideband suppressed carrier telephony has virtually replaced double-sideband amplitude-modulated telephony in the amateur service. The emission symbol is 2K70J3E, although there is some use of narrower and wider bandwidths. SSB is used on frequencies from 1.8 MHz through 47.2 GHz.
So we see where it comes from and there's not hard and fast number. Most radios people might try to use from the (Yaesu, Icom, Kenwood, Elecraft) have filtering to 2.7 kHz or less at -6dB anyway.