How can I retrieve the current node ID on a custom route?
Your route doesn't have a parameter called node, so therefore $node is null.

In order to make node available as a parameter you have to be on a route that has node as a parameter.

So to add the node parameter to your route, you can do like this:

rsvplist.form:
  path: '/rsvplist/{node}'
  defaults:
    _form: '\Drupal\rsvplist\Form\RSVPForm'
    _title: 'RSVP to this Event'
  requirements:
    _permission: 'access content'
    _entity_access: 'node.view'
    node: \d+


And then you can add the $node parameter to your buildForm method, like this:

public function buildForm(array $form, FormStateInterface $form_state, NodeInterface $node = NULL) {
  // Your form.
}


Since I added node: \d+ to your form, $node is now the loaded entity in the buildForm method.