Square Enix announced during their FFXVI special event yesterday that the demo for the game would launch on PS5 today, June 12.
The demo will cover a couple hours of the game's prologue, where your progress will carry over to the main game. After that portion, it will also present a special out-of-context battle section to show off the combat gameplay.
Don't forget even if you are only doing basic moves (no eikon) you have a lot more options than just mash square.
You have a closer with thrust/phoenix strike.
You can weave every single strike with triangle (timing is tight but doable) and get extra magic bursts to do more will damage.
You can combine abilities to bring enemies into the air (charged shot and especially aerial burning blade work well for this too) so you can do different combos on them there.
All that stuff is doable after you get abilities halfway through the demo. On the Eikonic challenge you have so many options that you can easily tear through stuff doing only a few combos, even on bosses!