There's no way to access the previous demo versions anymore, but there's a bunch of very early playable builds available here:
https://twitter.com/ULTRAKILLGame/status/1237532270937792513
And my youtube channel has a bunch of devlog videos showing the progress of the game's development:
https://www.youtube.com/playlist?list=PLFG57rtsH8Jso3qfd5eO6egyBtW3nyvir