Did Spain change its travel restrictions with respect to the United Kingdom?

Yes, the rules did change due to the end of the State of Emergency in June and the adoption of the EU Council recommendatuos effective to the 1st of July.

The Spanish Foreign Affairs was last updated in 2016, so is not helpfull.

So why the IATA site categories have only now changed (21st of July) can only be guessed at.

with the first category, it seemingly applies to all passengers from a given country;

Based on the first paragraph below, the situation was more restrictive: only residents of Spain were allowed.

with the second, it seemingly applies only to the residents of the country.

it should also apply to the other residents of the EU+ area and the recommended 14 other countries that have a reciprocity agreement.

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Entry rules in response to coronavirus (COVID-19)
Entry to Spain
The State of Emergency (“Estado de Alarma”) ended on 21 June. Spain’s borders are now open to European Union and Schengen-area countries. This means that British nationals no longer need to present a residency certificate to enter Spain.

The Spanish Government has confirmed that travellers arriving from the United Kingdom from 21 June are not required to self-isolate on arrival in Spain.

Restrictions on travellers from outside the European Union and the Schengen free-travel area will be lifted from 1 July, providing the countries they depart from have agreements with Spain.

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Sources:

• Entry requirements - Spain travel advice - GOV.UK
• Entry Requirements - Spanish Foreign Affairs Ministry