As per the FAQ
There is no demo of the game available at this time. If any early access or playtest periods come to be, they will be announced beforehand. If you've come across footage of people playing a "demo", you've probably seen an old version of the game that used to be available on itch.io. That version of the game is a different game and not representative of Tiny Combat Arena in its current state.
And the sticky
For the time being, I've unlisted the download since the version that was previously available is so far removed from the current incarnation that I'd rather it not cause confusion. I plan on making it available again at a later time, or if I can figure out some better way to distribute it while still making it very clear that it's not the same thing as the game on Steam.
The Steam version of the game isn't the "full version" it's a different game.