This would be a CPU command that works with the RAM controller rather than something you control yourself (kernels to my knowledge don’t talk directly to the controller beyond maybe some basic power management, if that).

There is a definite need to do hundreds of MB - the Linux kernel has a background thread that does nothing but zero out pages. What do you think happens to the GBs of RAM freed by closing Chrome? Once it’s made available in one spot, no reason others could use it (eg a hardened malloc implementation, etc).