I get the message "Secure Renegotiation IS NOT supported" if a TLS 1.3 cipher will be chosen at a TLS handshake with openssl s_client. Since TLS 1.3 does not support renegotiation at all I assume that this message can be ignored. Is my assumption correct?
Yes, since TLS1.3 forbids any renegotiation, supporting rfc5746 with it would be meaningless, and rfc8446 doesn't even mention the possibility.
Of course an endpoint that supports both 1.3 and lower versions should (in the 2119-capitalized sense) use 5746 for sessions that negotiate a lower version, and (AFAIK) OpenSSL does.