Here is a possible change (... to master, assuming it's good to support use case of RFC 8702):

diff --git a/cipher/keccak.c b/cipher/keccak.c
index 22c40302..76e08cb5 100644
--- a/cipher/keccak.c
+++ b/cipher/keccak.c
@@ -1630,8 +1630,8 @@ const gcry_md_spec_t _gcry_digest_spec_sha3_512 =
 const gcry_md_spec_t _gcry_digest_spec_shake128 =
   {
     GCRY_MD_SHAKE128, {0, 1},
-    "SHAKE128", shake128_asn, DIM (shake128_asn), oid_spec_shake128, 0,
-    shake128_init, keccak_write, keccak_final, NULL, keccak_extract,
+    "SHAKE128", shake128_asn, DIM (shake128_asn), oid_spec_shake128, 32,
+    shake128_init, keccak_write, keccak_final, keccak_read, keccak_extract,
     _gcry_shake128_hash_buffers,
     sizeof (KECCAK_CONTEXT),
     run_selftests
@@ -1639,8 +1639,8 @@ const gcry_md_spec_t _gcry_digest_spec_shake128 =
 const gcry_md_spec_t _gcry_digest_spec_shake256 =
   {
     GCRY_MD_SHAKE256, {0, 1},
-    "SHAKE256", shake256_asn, DIM (shake256_asn), oid_spec_shake256, 0,
-    shake256_init, keccak_write, keccak_final, NULL, keccak_extract,
+    "SHAKE256", shake256_asn, DIM (shake256_asn), oid_spec_shake256, 64,
+    shake256_init, keccak_write, keccak_final, keccak_read, keccak_extract,
     _gcry_shake256_hash_buffers,
     sizeof (KECCAK_CONTEXT),
     run_selftests

Reading RFC 8702, I realized that it defines the hash size in the use of CMS as: SHAKE128 : 32-byte SHAKE256 : 64-byte.

Applied rC8cdd0d353e19: cipher:pubkey: Check digest size which should not be zero. for 1.10.

Use Kleopatra which constructs the DN for you ;-).

I tested this with OpenPGP and 2.4.3-beta19 on Windows. Worked nicely.

I found this use case: RFC 8702
"Use of the SHAKE One-Way Hash Functions in the Cryptographic Message Syntax (CMS)": https://www.rfc-editor.org/rfc/rfc8702.html

Another possibility for digest&sign API: it is possible to determine the length of required hash function by the underlining field Fp of the curve in use. Then, use this length instead. It's better than to (try to) get the length by _gcry_md_get_algo_dlen (for SHAKE, it's undefined).

Fixed in both of master and 1.10 branch.

For libgcrypt, initially when the code was put, it made some sense.
Now, it's useless, so, let's simply remove the message.

Added: rC547dfb5aecc1: cipher:ecc: Add selftests for EdDSA.
Added: rC3ac2bba4a4b1: cipher:ecc: Implement PCT for EdDSA.

I have now disabled the rewriting in the 2.4 branch. Those who want to keep the old behaviour may add

could not trigger it with the described steps on windows

works for 4,1 GB, too.
(Tested with Gpg4win-4.2.0-beta346)

Move back to the backlog and trigger re-evalutation of priority (which was high).