Back in 2013 an alpha build of Half-Life called version 0.54, a version first distributed to game journalists in 1997, was released to the public. Its six chapters included one called The Office Warrens, which would eventually evolve into the finished game's fourth chapter. It served as the inspiration for the In Deep mod, which you can try an early version of now.
"In Deep takes you through the scrapped version of Half-Life from 1997," as the description says, "re-imagined and revamped. What awaits you is a surreal, fast-paced, and action-packed journey through the old Labs once again!"
The mod tweaks the alpha version by simplifying some of its more obtuse puzzles and making the combat feel a bit closer to that of the finished game. It's Half-Life as it might have been, if some of its early ideas had been polished rather than scrapped and replaced. You can download it from ModDB.
