Armor Stand Overhaul is a simple and lightweight data pack that brings the arms to armor stands by default!
You can summon an armor stand with arms with the ShowArms (/summon minecraft:armor_stand ~ ~ ~ {ShowArms:1b}) tag, but that only works on the ones you summon.
This data pack gives every armor stand, old or new, arms for a more refreshing look!
It is also well-optimized, because usually when you are giving something a tag (e.g. NoAI), if you are constantly doing it it will even tag the ones that have already had the specified tag, causing unnecessary lag. This data pack only tags armor stands that don't have arms, saving performance.
Enjoy!