File:Lowest bedrock generation in an extreme Custom world.png

The terrain generator can place bedrock as low as Y=−2048 in Custom worlds with extended min_y. Custom settings used:

{
	"bonus_chest": false,
	"dimensions": {
		"minecraft:overworld": {
			"type": {
				"logical_height": 2368,
				"infiniburn": "minecraft:infiniburn_overworld",
				"effects": "minecraft:overworld",
				"ambient_light": 0.0,
				"respawn_anchor_works": false,
				"has_raids": true,
				"min_y": -2048,
				"height": 2368,
				"natural": true,
				"coordinate_scale": 1.0,
				"piglin_safe": false,
				"bed_works": true,
				"has_skylight": true,
				"has_ceiling": false,
				"ultrawarm": false
			},
			"generator": {
				"biome_source": {
					"seed": 1,
					"large_biomes": false,
					"type": "minecraft:vanilla_layered"
				},
				"seed": 1,
				"settings": {
					"aquifers_enabled": true,
					"noise_caves_enabled": true,
					"deepslate_enabled": true,
					"bedrock_floor_position": 0,
					"sea_level": 63,
					"disable_mob_generation": false,
					"default_block": {
						"Name": "minecraft:stone"
					},
					"default_fluid": {
						"Properties": {
							"level": "0"
						},
						"Name": "minecraft:water"
					},
					"bedrock_roof_position": -10,
					"structures": {
						"stronghold": {
							"distance": 32,
							"spread": 3,
							"count": 128
						},
						"structures": {
							"minecraft:fortress": {
								"spacing": 27,
								"separation": 4,
								"salt": 30084232
							},
							"minecraft:swamp_hut": {
								"spacing": 32,
								"separation": 8,
								"salt": 14357620
							},
							"minecraft:ocean_ruin": {
								"spacing": 20,
								"separation": 8,
								"salt": 14357621
							},
							"minecraft:village": {
								"spacing": 32,
								"separation": 8,
								"salt": 10387312
							},
							"minecraft:desert_pyramid": {
								"spacing": 32,
								"separation": 8,
								"salt": 14357617
							},
							"minecraft:bastion_remnant": {
								"spacing": 27,
								"separation": 4,
								"salt": 30084232
							},
							"minecraft:shipwreck": {
								"spacing": 24,
								"separation": 4,
								"salt": 165745295
							},
							"minecraft:pillager_outpost": {
								"spacing": 32,
								"separation": 8,
								"salt": 165745296
							},
							"minecraft:ruined_portal": {
								"spacing": 40,
								"separation": 15,
								"salt": 34222645
							},
							"minecraft:buried_treasure": {
								"spacing": 1,
								"separation": 0,
								"salt": 0
							},
							"minecraft:jungle_pyramid": {
								"spacing": 32,
								"separation": 8,
								"salt": 14357619
							},
							"minecraft:igloo": {
								"spacing": 32,
								"separation": 8,
								"salt": 14357618
							},
							"minecraft:stronghold": {
								"spacing": 1,
								"separation": 0,
								"salt": 0
							},
							"minecraft:nether_fossil": {
								"spacing": 2,
								"separation": 1,
								"salt": 14357921
							},
							"minecraft:mansion": {
								"spacing": 80,
								"separation": 20,
								"salt": 10387319
							},
							"minecraft:endcity": {
								"spacing": 20,
								"separation": 11,
								"salt": 10387313
							},
							"minecraft:mineshaft": {
								"spacing": 1,
								"separation": 0,
								"salt": 0
							},
							"minecraft:monument": {
								"spacing": 32,
								"separation": 5,
								"salt": 10387313
							}
						}
					},
					"noise": {
						"simplex_surface_noise": true,
						"random_density_offset": true,
						"size_vertical": 2,
						"density_factor": 1.0,
						"density_offset": -0.46875,
						"top_slide": {
							"target": -10,
							"size": 3,
							"offset": 0
						},
						"bottom_slide": {
							"target": -30,
							"size": 0,
							"offset": 0
						},
						"size_horizontal": 1,
						"min_y": -2048,
						"height": 2368,
						"sampling": {
							"xz_scale": 0.9999999814507745,
							"y_scale": 0.9999999814507745,
							"xz_factor": 80.0,
							"y_factor": 160.0
						}
					}
				},
				"type": "minecraft:noise"
			}
		},
		"minecraft:the_nether": {
			"type": "minecraft:the_nether",
			"generator": {
				"biome_source": {
					"preset": "minecraft:nether",
					"seed": 1,
					"type": "minecraft:multi_noise"
				},
				"seed": 1,
				"settings": "minecraft:nether",
				"type": "minecraft:noise"
			}
		},
		"minecraft:the_end": {
			"type": "minecraft:the_end",
			"generator": {
				"biome_source": {
					"seed": 1,
					"type": "minecraft:the_end"
				},
				"seed": 1,
				"settings": "minecraft:end",
				"type": "minecraft:noise"
			}
		}
	},
	"seed": 1,
	"generate_features": true
}