Machine Counter-Inhibit CSR (mcountinhibit) not implemented on U740?

I don’t see any mentioning in SiFive FU740-C000 Manual.

freedom-e-sdk has bsp files with register lists in them. This is the best place to check for the list of supported, as this is generated directly from the hardware source code.
I don’t see mcountinhibit in the file.

All CSRs are optional, so there is no requirement to implement any particular CSR.
Unless of course we need it to boot linux.

That’s not true, some are mandatory in M-mode (though it gets fuzzy if you’re allowed to emulate them), and others are mandatory under various other conditions (e.g. medeleg/mideleg are only optional if you lack both S-mode and the N extension). mcountinhibit happens to be one that’s optional though.