The SiFive U74 core in the RPi-format SBC is RV64GC with the most useful parts of Bitmanip: Zba and Zbb
From a little bit of googling, I found out that the Milk-V Duo's SoC has the T-Head XuanTie C906 core.
Both the C920 and C906 are RV64GC, and also implement the 0.7.1 draft of the Vector extension (!) extended with 16-bit FP support.
No Bitmanip, but instead T-head's own "thead" extension which has a lot of overlap with Zba and Zbb.
BTW. Because there are several 64-bit RISC-V microcontrollers with relatively little memory on the way, there's a special Linux profile in development that uses 32-bit addressing to save memory.