Java Edition:Resistance VI
Resistance VI allows you to be invulnerable to most dangerous factors of survival mode in versions 18w07a - 18w15a. There was one main way to obtain Resistance VI through the Strong Potion of the Turtle Master. You could even get the infinite version of the effect in 23w13a_or_b.
Applications
Resistance VI is extremely useful, as it makes the player immune to all damage except for void and starvation. It adds 20% of Resistance per level. This means that a Resistance level over V gives over 100% resistance, which is complete invulnerability.
Obtaining
Strong Potion of the Turtle Master
Resistance VI was available through a Strong Potion of the Turtle Master in versions 18w07a through 18w15a. This was changed to Resistance IV in 18w16a.
This is a temporary discontinued feature, as the effect only lasts for 60 seconds. This effect can also be applied to any other mob that accepts potion effects, via a Strong Splash Potion of the Turtle Master.
23w13a_or_b Vote Rule
In the April Fools snapshot 23w13a_or_b, you could vote to get infinite duration effects. These effects could be random, so you could possibly get infinite Resistance VI.