Não posso comentar sobre SEO, mas o uso típico da resposta 204 é não atualizar a página atual. Provavelmente é mais apropriado com um pedido diferente de GET (como POST ou DELETE):
Trecho de HTTP / 1.1: definições do código de status :
10.2.5 204 No Content
The server has fulfilled the request but does not need to return an
entity-body, and might want to return updated metainformation. The
response MAY include new or updated metainformation in the form of
entity-headers, which if present SHOULD be associated with the
requested variant.
If the client is a user agent, it SHOULD NOT change its document view
from that which caused the request to be sent. This response is
primarily intended to allow input for actions to take place without
causing a change to the user agent's active document view, although
any new or updated metainformation SHOULD be applied to the document
currently in the user agent's active view.
The 204 response MUST NOT include a message-body, and thus is always
terminated by the first empty line after the header fields.