Disclaimer for devs: Do NOT implement your fixes based on my implementation, my context is limited to mixins and your mod's business logic affinity.

Leaks

• emi
  • [1.0.21,)
    • Clear EmiHistory when client player cloned
• minecraft
  • 1.20.1
    • Clears Ender Dragon entity from model after usage (disabled when modernfix is loaded)
    • Prevents putting entity on constant ItemStack.EMPTY, clear entities on crosshair/hitresult level update, clears old ItemStack#entityRepresentation from tracked itemstacks at SynchedEntityData
    • Adds a check of last damage to clear removed entities from it
    • Clears temporary entity tick list after done looping
• cyclopscore
  • [1.19.1,)
    • Updates DelegatingDynamicItemAndBlockModel#world on client level update
• blue_skies
  • [1.3.31,)
    • Clears ClientPacketListener#blue_skies$lastRidden mixin field and updates SkiesClientEvents#dungeonAmbientSoundHandler on server stopped/client player clone
• mousetweaks
  • [2.25.1,)
    • Clears Main#openScreen/handler/oldSelectedSlot on client player clone
• ae2wtlib
  • [15.2.3,)
    • Clears client cloned player from CraftingTerminalHandler#players
    • Clears AE2WTLibCreativeTab#items before adding items
    • Clears server cloned players from CraftingTerminalHandler#players
• toolbelt
  • [1.20.0,)
    • Use reviveCaps instead of revive and adds invalidateCaps at BeltExtensionSlot.EventHandlers#playerClone
• ftblibrary
  • [2001.2.2,)
    • Clears BLANK_GUI#prevScreen on client player clone
• tfcthermaldeposits
  • [1.3.2,)
    • Clears TDForgeEventHandler#worldLevel on server stop
• geckolib
  • [4.4.8,)
    • Uses WeakReferences to prevent memory leaks on GeoModel#applyMolangQueries()
• aether
  • [1.20.1-1.4.2,)
    • Reworked DroppedItemCapability to not leak entities
• jei
  • [15.4,15.5)
    • Clears RecipesGui#recipeTransferButtons on client player update
  • [15.8.2.24,)
    • Updates RecipeTransferButton#player and RecipeTransferButton#parentContainer on client player clone using RecipeTransferButton#update()
• forbidden_arcanus
  • [1.20.1-2.2.0-beta1,)
    • Adds an invalidateCaps that was missing on PlayerEvents#onPlayerClone
• difficultylock
  • [4.1,)
    • Clears Util variables on server stop
• easy_villagers
  • [1.20.1-1.1.4,)
    • Clears ItemTileEntityCache#CACHE and VillagerItem#cachedVillagers cache on client level update
• findme
  • [3.1.0,)
    • Clears FindMeModClient#lastRenderedStack on client level update
• flywheel
  • [0.6.9-4,)
    • Prevents old world to be added to the world list via WorldAttached#put()
• iceberg
  • [1.1.10,)
    • Unload client level from EntityCollector#wrappedLevelsMap
    • Clears CustomItemRenderer#armorStand/horse/entity on client level update
  • [1.1.25,)
    • Clears CustomItemRenderer#blockEntity on client level update
• curios
  • [5.9.1,)
    • Use reviveCaps instead of revive and invalidate on CuriosEventHandler#playerClone
• create
  • [0.5.1.c,)
    • Clears last damage on clone from ExtendoGripItem#lastActiveDamageSource
• journeymap
  • [5.9.21,)
    • Clears EntityDistanceComparator#player and EntityDTODistanceComparator#player after usage
• emi_loot
  • [0.6.5,)
    • Recreates entities at EntityEmiStack on client level update
• nuclearcraft
  • [1.0.0,)
    • Clear TooltipHandler#processedEvent on client player clone
• citadel
  • [2.5.4,)
    • Clears server from CitadelServerData#dataMap on stop
• travelersbackpack
  • [9.1.0,)
    • Use reviveCaps instead of revive and adds invalidateCaps at ForgeEventHandler#playerClone
  • [9.1.16,)
    • Clears entity from BackpackLayerModel#livingEntity
• badpackets
  • [0.4.1,)
    • Clears ChannelRegistry#handlers map on packet close
    • Clears ChannelRegistry#handlers map on server stopped
• irons_spellbooks
  • [1.20.1-3.4.0,)
    • Clears SpellBarOverlay#lastSelection and ClientMagicData#spellSelectionManager on client clone
    • Update SyncedSpellData#livingEntity on player clone
• railcraft
  • [1.1.2,1.1.6]
    • Clears level from ChargeProviderImpl.DISTRIBUTION#networks on level unload
• tombstone
  • [8.5.0,8.8.6]
    • Clears LivingEntityRenderer#entity added via mixin on client level update
• pneumaticcraft
  • [6.0.15,)
    • Clears ArmorMainScreen#upgradeOptions on client player clone
    • Unregister drones from MinecraftForge event via Entity#onRemovedFromWorld()
• minecolonies
  • [1.20.1-1.1.647-beta,)
    • Recreates citizens and entities on level update
• architectury
  • [9.0.8,)
    • Update NetworkManagerImpl#clientReceivables on player clone
• ldlib
  • [1.0.25.p,)
    • Refactor a lambda that leaks level and updates ModularUI#entityPlayer on player update
• occultism
  • [1.80.7,)
    • Adds a missing invalidateCaps at OccultismCapabilities#onPlayerClone on player clone
• betterf3
  • [7.0.2,)
    • Reworked LocationModule#update() to prevent leak of current chunk
• jeresources
  • [1.4.0.247,)
    • Uses Minecraft#level to create entities on MobTableBuilder#add/addSheep and updates MobTableBuilder#level on level update
• ars_nouveau
  • [4.0.0,4.12.4]
    • Uses reviveCaps instead of revive on CapabilityRegistry.EventHandler#playerClone()
    • Rework lambda to not leak level on CasterTomeRegistry#reloadTomeData
• smallships
  • [2.0.0-b1.4,)
    • Clears Smallships mixin variables ChunkMap#serverPlayer/list after usage
• forge
  • [47.2,)
    • Make FakePlayers stop listening to advancements
    • Clears packet listener on server stop for FakePlayers
    • Regenerate listeners cache on server stopped
• twilightforest
  • [4.3.2508,)
    • Clears EntityRenderer#ENTITY_MAP and Hydra entity from HydraModel#hydra on level update
• corpse
  • [1.20.1-1.0.3,)
    • Clears CorpseRenderer#players/skeletons on client level update
• createaddition
  • [1.20.1-1.0.0b,1.20.1-1.2.3]
    • Clears level from EnergyNetworkManager#instances on level unload

Feature (config/alltheleaks.json)

• minecraft
  • 1.20.1
    • Deduplicates VANILLA ingredients to reduce memory usage - (default: OFF)
    • Deduplicates ResourceLocations, should be tested to check if it is worth for your case - (default: OFF)
• jei
  • [15.4.0.9,)
    • Prevents JEI from creating a search tree nodes for items that are ignored on config/jei/blacklist.cfg
      
      Note: This will prevent you from finding it at Creative Tab if using ModernFix search tree backed by JEI! - (default: OFF)

Fix

• modernfix
  • [5.0.0,)
    • Replace modernfix RL deduplicate feature if needed

Metrics (Dev Only -Dalltheleaks.indev=true)

• jei
  • [15.4.0.9,)
    • Adds metrics to see which JEI Plugins are creating more ItemStacks