Is it possible to get demographics data like age, gender, etc.. in the GA4 S2S pixel?

Explicitly no, you cannot. Personal demographics are coming from Google accounts so you can track them only with Google Signals and you can use it only with the Client-side implementation. However,  language and location data are coming from IP address and browser language so these could be extrapolated from the IP-Address, if it's being consensually tracked with no anonymization.