Per https://en.wikipedia.org/wiki/Transistor_count, even an 8051 has 50K transistors, which reinforces my claim that 26K really doesn't seem like a big ask for an MCU core. Whether that means a barrel shifter is worth it or not is a totally orthogonal question, of course.

(Although I do have to eat my words here - I didn't check that Wikipedia page, and it does actually list a ~6K RISC-V core! It's an experimental academic prototype "made from a two-dimensional material [...] crafted from molybdenum disulfide"; I don't know if that construction might allow for a more efficient transistor count and it's totally impractical - 1KHz clock speed, 1-bit ALU, etc. - for almost any purpose, but it is technically a RISC-V implementation significantly smaller than 26K)

> There's reason RV32E and RV64E, with half the registers, are a thing. RV32I/RV64I isn't small enough.

This is actually kind of counter to your point. The really tiny micro-controllers from the 80s only had 224 bits of registers. RV32E is at least twice that (16 registers*32 bits), and modern mcus generally use 2-4kbs of sram, so the overhead of a 32 bit barrel shifter is pretty minimal.