This code

if ((only_mainkey && main_pk->version < 4)
     || (!only_mainkey && sub_pk->version < 4))
{
  log_info (_("You can't change the expiration date of a v3 key\n"));
  return gpg_error (GPG_ERR_LEGACY_KEY);
}

should have kicked in but it didn't for your key. Would you mind to check that key using --list-packets to view the versions of the the public key and public sub key packets? (gpg --export YOURKEY | gpg --list-packets | less)

Here is the output of --list-packets of the offending key, anonymised:

1. off=0 ctb=99 tag=6 hlen=3 plen=418 :public key packet: version 4, algo 17, created 985690138, expires 0 pkey[0]: [1024 bits] pkey[1]: [160 bits] pkey[2]: [1024 bits] pkey[3]: [1023 bits] keyid: <KEY_ID>
2. off=421 ctb=b4 tag=13 hlen=2 plen=35 :user ID packet: "XXXXXXXXXXXXX"
3. off=458 ctb=88 tag=2 hlen=2 plen=120 :signature packet: algo 17, keyid <KEY_ID> version 4, created 1629537425, md5len 0, sigclass 0x13 digest algo 2, begin of digest a8 22 hashed subpkt 33 len 21 (issuer fpr v4 <XXXXXXXXXXXXXX><KEY_ID>) hashed subpkt 2 len 4 (sig created 2021-08-21) hashed subpkt 27 len 1 (key flags: 23) hashed subpkt 11 len 4 (pref-sym-algos: 9 8 7 2) hashed subpkt 21 len 5 (pref-hash-algos: 8 9 10 11 2) hashed subpkt 22 len 3 (pref-zip-algos: 2 3 1) hashed subpkt 30 len 1 (features: 01) hashed subpkt 23 len 1 (keyserver preferences: 80) subpkt 16 len 8 (issuer key ID <KEY_ID>) data: [158 bits] data: [159 bits]
4. off=580 ctb=b9 tag=14 hlen=3 plen=525 :public sub key packet: version 4, algo 16, created 985690139, expires 0 pkey[0]: [2048 bits] pkey[1]: [2 bits] pkey[2]: [2046 bits] keyid: YYYYYYYYYYYYYYY
5. off=1108 ctb=88 tag=2 hlen=2 plen=63 :signature packet: algo 17, keyid <KEY_ID> version 3, created 985690139, md5len 5, sigclass 0x18 digest algo 2, begin of digest 94 e5 data: [159 bits] data: [156 bits]

Thanks for looking into this

I think that this commit rG8fd150b05b74: gpg: Remove all support for v3 keys and always create v4-signatures. matters.

Perhaps, something like this is needed to avoid generating v3 signature:

diff --git a/g10/sign.c b/g10/sign.c
index 2ab76c99b..8547a3ab6 100644
--- a/g10/sign.c
+++ b/g10/sign.c
@@ -1928,6 +1928,10 @@ update_keysig_packet (ctrl_t ctrl,
   /* Create a new signature packet.  */
   sig = copy_signature (NULL, orig_sig);
 
+  /* Don't generate version 3 signature, but newer.  */
+  if (sig->version == 3)
+    sig->version = pk->version;
+
   sig->digest_algo = digest_algo;
 
   /* We need to create a new timestamp so that new sig expiration

Not applying the change to GnuPG 2.2, users can use GnuPG 2.3 for that.