Originally Posted by
Jipe
The problem is that a cassette that starts with a smallest cog of 11t is not good for small wheel bikes, the gear inch it gives with a usual 52 or 53t chainring and 52/53x11 is too short and the big cogs above 42 are completely useless. Using very big 60t or more chainring is not a good solution because they aren't standard and do not fit on many folding bikes.
This is the reason why bike manufacturers like Moulton, Riese & Müller and others use special cassettes with a smallest cog of 10t or 9t.
Shimano has only one solution that accept a 10t cog, its the latest MTB 12s Hyperglide+ which is only compatible with Shimano microspline hub and only in 10-45 or 10-51 (or even bigger from third party).
Shimano had the Capreo drivetrain specially made for small wheels bike but its obsolete (cassette is max 9-28) and anyway discontinued from Shimano and n spare parts available anymore.
Therefore the choice of SRAM XD or XDR rear hub that accepts cassettes starting with a 9t cog, for which there are several cassettes options that fits very well for small wheel bikes with a wide enough gear inch range to avoid the need of a double chainring and that in 11s are compatible with SRAM, Shimano and even Campagnolo derailleurs.
The picture I put of my Birdy show such a solution : XDR rear hub, 11s 9-34t 378% range cassette and Ultegra RD-R8000-GS medium cage rear derailleur (and for a frame that accepts it, there is also a 9-39t cassette 11s for XD/XDR rear hub usuable with a Shimano GRX derailleur).
I think you need to speak for yourself. For many, including myself, a 11t smallest cog and cogs beyond 42t are useful and adequate. Racing and speed may be your call. It is not not mine. Your terrain and uses are not mine terrain and uses. Highest point in Belgium is less than 700m. I was born at 1,600m. Highest point here is 3,100 and it's volcanic so there are steep river ravines to traverse everywhere. We need not discuss, of course, the great inefficiency and rapid wear of 9t cogs.