Earlier quoted context omitted.
Indeed, it is not service to EU users which is governed by GDPR, it is data processing of data subjects in the EU. Obviously the data processing as defined by the GDPR doesn't stop simply because the service stops since storage is considered processing. Seems to me Instapaper painted a big target on their chest: "We're not compliant, and we're going to give EU users the middle finger in the meanwhile." Whereas, their…
So if someone traveling in the EU gets GDPR protections, does someone traveling in the US lose GDPR protections? Are the GDPR protections only for the data that was collected while someone was in the EU or for all data once they've traveled to the EU once?
Roughly speaking, yes. E.g. a Brazilian business not specifically targeting transactions within the EU or EU nationals doesn't come under the GDPR. Conversely, an EU business basically has to apply the GDPR to any data collected inside the Union, regardless of the individual's nationality. (Obviously, this gets messier online, but if the currency was e.g. Euros and the company is in Belgium, probably covered.)
Having said that, you might have trouble as a non-EU resident to get the data protection agencies to care for trivial, individual cases. (My opinion as a European, not a fact.)