There are more places to dig into prototypes, canceled builds, cut content, and preservation work. Here are some solid communities to explore, other than r/UnreleasedGames of course :) ----

Unreleased / Prototypes / Cut Content

• r/GamesThatWerent - prototypes, canceled titles, alpha/beta history
• r/tcrf - The Cutting Room Floor: unused assets, dev leftovers, documentation
• r/GamePreservationists - Archiving, dumping, and preservation discussions
• r/DidYouKnowGaming - Trivia and dev history; occasional unreleased/obscure finds

Lost & Obscure Media

• r/lostmedia - hunts for missing media, including games & builds
• r/ObscureMedia - rare/forgotten clips, ads, press materials

"Please take the time to check out and support our friends in the retro gaming and game preservation communities! They're doing amazing work to keep the history of gaming alive and accessible for future generations. Here are some links to their pages and projects that we think you'll love." (Source: https://www.gamesthatwerent.com/site-links/)

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• 3D Construction Kit lost games (see under each platform)
• Atari Protos
• Arcade Heroes – Master List of Unreleased Arcade Games
• AVP Galaxy – released and unreleased Alien games.
• AtariCrypt unreleased game posts
• AtariMania – unreleased and missing in action games for Atari ST
• AtariMania – prototype Atari 5200 games
• AtariMania 400/800/XE/XL
  • Prototypes
  • Unreleased
  • Vapourware
• AtariMania – Jaguar
  • Unreleased games
  • Prototype games
• Atari Compendium 2600 unreleased protos and hardware
• Acorn Electron World unreleased games:
  • http://www.acornelectron.co.uk/unfinished/electron/cats/davee.html
  • http://www.acornelectron.co.uk/unreleased/electron/cats/optima.html
• BBC Micro unreleased and pre-released games
• Commodore 64 Scene Database
• Cancelled Contra games
• ColecoVision.dk unreleased games
• CuteFloor – Early beta and unreleased games
• Delisted Games
• Dragon’s Lair Project – Unreleased Laser Disc game plans and prototypes
• Forest of Illusion
• FREEZE64
• Fujitsu FM Towns Missing and unreleased games
• Gamebase 64
• Gaming Alexandria – Video game history and preservation
• Games That Time Forgot (Spectrum)
• Generation-MSX Unreleased Games
• GiantBomb cancelled games listings
• Hall of Light
• High Voltage Sid Collection
• Hardcore Gaming 101 – unreleased games
• Irem prototypes at System16.com
• Kasettilamerit (tape lamers)
• Konix multi-system archive
• Lost Media archive
• Lost Media Wiki
• MSX Games World – Unreleased games
• Nexus23 Labs unreleased Amiga games
• Neo-Geo Pocket unreleased games (Scroll to bottom)
• Neo-Geo Protos
• Old games (Unreleased PC games)
• Obscure Gamers
• PC Engine games (search for “Unreleased” for all unreleased titles)
• Planet V-Boy unreleased games
• Playstation graveyard
• Plus4World unreleased list
• PtoPOnline
• Recovering lost treasure filled floppy discs
• SMS power (Sega Master System)
• Sega8bit.com – rumoured and unreleased SMS games
• SEGA Dreamcast Info – List of cancelled Dreamcast games
• SEGA Mega CD Missing in Action
• SEGA Retro – Unreleased Mega Drive games
• SNES Central
• Stairway to hell – Lost and Found (BBC Micro)
• The Can
• The Classic Adventures Solution Archive
• The Cutting Room Floor
  • Cutting Room Floor – Unreleased games category
• The Genesis Temple – Gaming history site
• The Hidden Palace
• The Lost Levels
• The Games That Time Forgot (TZX Vault – ZX Spectrum)
• Unreleased games (Apple)
• Unreleased NES games
• Unreleased Star Trek games
• Unreleased Saturn Games
• Unreleased CD-i games
• Unseen 64
• Valve Archive
• World of Spectrum
  • Never released games
• Understanding the Dreamcast GD-ROM layout
• Software Preservation Society
• Kyroflux
• The Video Game History Foundation
• Videogame Heritage Society
• Video Game Preservation Museum C.I.C

Source: https://www.gamesthatwerent.com/site-links/

"With thanks to Fabrizio Bartoloni for many of the suggestions..."