Netplan does not provide "key" parameter to GRE tunnel configuration via systemd-networkd

The following example should configure a GRE tunnel between 172.16.4.2 and 172.16.1.2, with key 33490175. The effect should be like manually configured with: sudo ip tunnel add test1 mode gre local 172.16.4.2 remote 172.16.1.2 key 33490175.

However, the following Netplan configuration fails under Ubuntu 22.04 (latest development version, March 25, 2022):

network:
  version: 2
  renderer: networkd
  tunnels:
    gre1-255-255:
      mode: gre
      ttl: 255
      local: 172.16.4.2
      remote: 172.16.1.2
      key: 33490175
      addresses:
        - 172.20.82.94/30

Result of "sudo netplan generate": Error in network definition: gre1-255-255: 'input-key' is not required for this tunnel type

Note: There should be no input and output keys. There is just one single key!

Trying to change the configuration:
     keys:
        input: 33490175
     # key: 33490175

Result of "sudo netplan generate": Error in network definition: gre1-255-255: 'input-key' is not required for this tunnel type

Some investigation into the sources of Netplan (src/validation.c -> https://github.com/canonical/netplan/blob/main/src/validation.c):

static gboolean
validate_tunnel_backend_rules(NetplanNetDefinition* nd, yaml_node_t* node, GError** error)
{
    /* Backend-specific validation rules for tunnels */
    switch (nd->backend) {
        case NETPLAN_BACKEND_NETWORKD:
            switch (nd->tunnel.mode) {
                case NETPLAN_TUNNEL_MODE_VTI:
                case NETPLAN_TUNNEL_MODE_VTI6:
                case NETPLAN_TUNNEL_MODE_WIREGUARD:
                    break;

                /* TODO: Remove this exception and fix ISATAP handling with the
                 * networkd backend.
                 * systemd-networkd has grown ISATAP support in 918049a.
                 */
                case NETPLAN_TUNNEL_MODE_ISATAP:
                    return yaml_error(node, error,
                                      "%s: %s tunnel mode is not supported by networkd",
                                      nd->id,
                                      g_ascii_strup(tunnel_mode_to_string(nd->tunnel.mode), -1));
                    break;

                default:
                    if (nd->tunnel.input_key)
                        return yaml_error(node, error, "%s: 'input-key' is not required for this tunnel type", nd->id);
                    if (nd->tunnel.output_key)
                        return yaml_error(node, error, "%s: 'output-key' is not required for this tunnel type", nd->id);
                    break;
            }
            break;

It seems that the cases NETPLAN_TUNNEL_MODE_GRE and NETPLAN_TUNNEL_MODE_IP6GRE are missing for systemd-networkd. According to https://github.com/systemd/systemd/issues/12144, systemd-networkd should support configuration with keys since ca. 2 years.