Given that 1e fighters likely had plenty of magic bonuses to their saves (from magic armour or rings), that 20th level fighter was probably only failing on a roll of 1.
Most of the time a 20th level fighter in 2024 will succeed on a 1. With indomitable, a 1 on the die is a 21 then you add proficiency, ability bonus or any magic item or spell buffs.
With Mage Slayer, Indomitable and Heroic Warrior, I don't think the fighter in the campaign I just finished failed a single save that mattered past 15th level.