Despite not being formally announced, the Ryzen 9 4900U has been benchmarked on 3DMark. Lenovo has been caught stating that the Yoga Slim 7 would support up to an “AMD Ryzen 9 4900U”, since then it reverted its UK product listing which is up to “AMD Ryzen 7” processors. According to this new 3DMark listing, the Ryzen 9 4900U has a maximum boost clock of 4.3 GHz, which is 100 MHz higher than the Ryzen 7 4800U. This 8 core processor also supports SMT, allowing it to execute up to 16 threads simultaneously. 3DMark reports that the Ryzen 9 4900U has a 1.8 GHz base clock too, but gives no details about TDP or the type of Radeon Graphics with which it has been paired.
According to @uzzi38, AMD rebranded the Ryzen 9 4900U to the Ryzen 7 4800U while we cannot confirm this. If this is true, the Ryzen 7 4800U can reach up to 4.3 GHz, 100 MHz above its advertised value. So, while there may be some Ryzen 9 4900U versions of the Yoga Slim 7 out there, but they might come to market as Ryzen 7 4800U models, instead. The lack of a Ryzen 9 U-series APU does seem like a lost opportunity.