So if you want to interpret "abandoning the class" as "losing abilities", which abilities do you lose, exactly? ASI's are class abilities according to the chart, do you lose those? Do they lose Fighting Style? Extra Attack? You gain 1d10 Hit Dice per paladin level, according to page 84. Do I lose all my proficiencies granted by my class?
Strange that if this was ever the intent, that the PHB couldn't be bothered to outline which abilities should be lost by "abandoning" the class, instead of forcing the DM to make a ruling about each one. Or does a classless character become an NPC, like the Soldier or Gladiator? Do you stop being able to play your character entirely because you violated your Oath?
If that was the case, the PHB could certainly have explained these things. Or the DMG could have a section about it.
The 3e PHB had a section for classes that could "lose" their abilities in the class description. If there's no guideline explaining how to massively rewrite a character due to failing to meet some obligation, I'm going to go with Occam's Razor on this one and assume the simplest version; ie, you have to change your Oath/God/Patron or start taking levels in some other class.