default or selected kernelversion does not have config value CONFIG_CRYPTO_RMD256.
Result is shown for kernelversion 6.9.9

RIPEMD-256 digest algorithm

modulename: rmd256.ko

configname: CONFIG_CRYPTO_RMD256

Linux Kernel Configuration
└─>Cryptographic API
└─>RIPEMD-256 digest algorithm
In linux kernel since version 2.6.27 (release Date: 2008-10-09)  
RIPEMD-256 is an optional extension of RIPEMD-128 with a
256 bit hash. It is intended for applications that require
longer hash-results, without needing a larger security level
(than RIPEMD-128).

Developed by Hans Dobbertin, Antoon Bosselaers and Bart Preneel.
See <http://homes.esat.kuleuven.be/~bosselae/ripemd160.html>

source code: