Started a Baldur's Gate 3 playthrough (I pirated it because of course), and decided to roll a Paladin, as to roleplay a class struggle oriented fighter against oppression, next thing I know I'm stuck with a whole bunch of silly ass moral codes and liberal bullshit.
In the end the aristocratic ass elf that you can get as a companion is doing all the hard work not to break the RP of my main character and I'm hardly using it for the fun things (assassinating class enemies).
SMH
In 5e rules-as-written it's pretty flexible.
The tenets of the Oath of Devotion (closest thing to a traditional paladin)
I'd say freeing slaves falls under Compassion for "protect the weak and punish those who threaten them" and as for slavery being legal I'd respond by pointing out Duty where "obey those who have just authority over you" by saying any system supporting slavery is not just.
Even apart from that, there's not really any alignment in 5e. It exists as a way to describe your character and a very small number of magic items require specific alignments. Spells like Protection from Evil and Good work against extraplanar beings like celestials and friends rather than things having good or evil alignment like they did in past editions.
All that to say if anyone tried to tell me my paladin broke their oath by freeing a slave and wouldn't listen to reason I'd probably leave their game. Oathbreaker paladins have no tenets or oath but just read this fucking description and tell me it sounds like someone who freed slaves lol