Musician Pegasus DAC claims 32bit/1536kHz with STM32F446 USB receiver. There are several projects on diyaudio for STM32 UAC2 device firmware, perhaps their authors (e.g. @bohrok2610 , @slerpxcq https://github.com/slerpxcq/openuac2_fw ) would tell you more.
1536k means the I2S data line will run at 100MHz.
Also UAC2 will need multiple transactions/high bandwidth per USB2 microframe - 1536 * 4 * 2 / 8 = 1536 bytes per microframe which requires two isochronous transactions (1024 bytes max) per microframe. E.g. iOS devices do not support this https://forum.rme-audio.de/viewtopic.php?pid=178246#p178246
1536k means the I2S data line will run at 100MHz.
Also UAC2 will need multiple transactions/high bandwidth per USB2 microframe - 1536 * 4 * 2 / 8 = 1536 bytes per microframe which requires two isochronous transactions (1024 bytes max) per microframe. E.g. iOS devices do not support this https://forum.rme-audio.de/viewtopic.php?pid=178246#p178246
It is quite possible to run STM32H7 UAC2 at 32bit/1536. The challenge is that I2S timings will be very tight and would require careful tuning at USB-I2S board. E.g. my boards have fast I2S isolators but even those may struggle at that rate. And not many DACs or ADCs support 1536k.
