Java Edition:Poison IV
From 13w36a-1 (1.7 snapshot) until 20w45a (1.17 snapshot), poison IV could be obtained on players and many mobs. The poison effect on any mob will wear off quickly. As a result, it is a Temporary Discontinued Feature.
Obtaining
Pufferfish
Pufferfish would give the poison IV effect to any entity that ate them from 13w36a (1.7 snapshot) to 20w45a (1.17 snapshot). The consumption inflicted 60 seconds of poison IV. However, only the player and the fox can consume the pufferfish and consequently obtain this effect.
Zones Dimension
In 20w14∞, certain dimensions (including the "zones" dimension) contained leftovers, which are blocks with villager trade particles that would inflict blindness II and poison IV for 3 seconds each. This effect lasts far shorter but can be applied to any mob that walked into the zone. However, it was difficult to keep these mobs because of the duration. The mob can be pushed through a portal (end or nether) and the world should be exited to unload the chunks before the effect runs out, or the mob can be kept in unloaded chunks.
Voting for the Poison IV Effect
In the April Fools snapshot 23w13a_or_b you could vote to get infinite duration effects. These effects could be random, so you could get infinite Poison IV or higher.
Gallery
A player with the poison 4 effect for 50 seconds from a pufferfish.
