In today's Nintendo Direct, it was announced that there is a full game trial version of ARMS available for Nintendo Switch Online users that will work from today, through April 6th
*Link above is for game info. Must go to the eShop on the Switch console to download the Trial version of ARMS, not the Demo above.
Thanks to Burndrive
for the clarification.
More info in the Direct Mini: https://youtu.be/ubRf4zyEvG4