Had the S21 for a while since the great deal on Ozb. Couldn't find it for a while. Only been charging and using it very rarely. Does this affect battery life? Does this shorten it? Not frequently using the phone? the keyboard is too small is why I don't like using it. I've notice the battery life is very short. Is this a design fault? Or me not charging it and not using it much?
TLDR: would not charging and not frequently using the phone shorten the battery life?
Yes if you leave it plugged in all the time. Sitting at max capacity can degrade lithium batteries. There is a setting in samsung phones to stop charging at 85% to mitigate this. You would only see an impact from this if it was almost always on the charger for a long time.
Settings>battery & device care>battery>protect battery.