Unfortunately SE scrapped the old cross-class ability system when the Stormblood expansion was launched, which indeed means you can no longer equip spells like Cure on PLD (PLD does get it's own potent healing spell called 'Clemency' though, but you have to wait til level 58 for that). Instead, we have what are called 'Role Abilities' that are automatically learned based on what role your class or Job is and can be used by any class or Job of that role. As a result we lost a lot of abilities but gained some new ones as well, plus at least one PLD ability was renamed and had it's level learned greatly reduced (Shield Oath became 'Iron Will' and is now learned by GLA at a mere level 10). This official page explains the Role Ability system and what abilities you can equip for GLA and PLD.

As for Ragnarok being dead, I still can't understand why Ul'dah would appear empty (retainers don't show up physically anymore since 1.0 ended), as there should at least be a few people still around. All I can think of is you must have indeed been playing during the server's quieter times as I think it would be more lively around 6-12pm GMT or thereabouts.