@achow101 noted we don't reject them therefore they must be silently accepted. This is not part of the specs, we should remove them. Also cc @sipa.

I can take care of this.
auto slash_split = Split(origin_split[0].subspan(1), '/');
    if (slash_split[0].size() != 8) {
        error = strprintf("Fingerprint is not 4 bytes (%u characters instead of 8 characters)", slash_split[0].size());
        return nullptr;
    }
    std::string fpr_hex = std::string(slash_split[0].begin(), slash_split[0].end());
    if (!IsHex(fpr_hex)) {
        error = strprintf("Fingerprint '%s' is not hex", fpr_hex);
        return nullptr;
    }
    auto fpr_bytes = ParseHex(fpr_hex);
    KeyOriginInfo info;
    static_assert(sizeof(info.fingerprint) == 4, "Fingerprint must be 4 bytes");
    assert(fpr_bytes.size() == 4);
    std::copy(fpr_bytes.begin(), fpr_bytes.end(), info.fingerprint);
    if (!ParseKeyPath(slash_split, info.path, apostrophe, error)) return nullptr;
    auto provider = ParsePubkeyInner(key_exp_index, origin_split[1], ctx, out, apostrophe, error);
    if (!provider) return nullptr;
    return std::make_unique<OriginPubkeyProvider>(key_exp_index, std::move(info), std::move(provider), apostrophe);
}
std::unique_ptr<PubkeyProvider> InferPubkey(const CPubKey& pubkey, ParseScriptContext, const SigningProvider& provider)
{
    std::unique_ptr<PubkeyProvider> key_provider = std::make_unique<ConstPubkeyProvider>(0, pubkey, false);
    KeyOriginInfo info;
    if (provider.GetKeyOrigin(pubkey.GetID(), info)) {
        return std::make_unique<OriginPubkeyProvider>(0, std::move(info), std::move(key_provider), false);
    }
    return key_provider;
}
std::unique_ptr<PubkeyProvider> InferXOnlyPubkey(const XOnlyPubKey& xkey, ParseScriptContext ctx, const SigningProvider& provider)