I've got decades of DM time, and the obvious intent of feeblemind is to effectively disable the target and make them completely helpless. As a plot device in your storyline, it is meant to frustrate your player and challenge the group to find the solution... but never to dis-engage the player so they no longer want to play the game. So it is 100% up to you the DM to be devious by introducing your feeblemind event/trap, yet also give hope and a way for the party to reverse the effects. True to 2e/3e fashion, it leaves much to the discretion, imagination, creativity and arguably intelligence (lol) of the DM and player.
If your question is "what would a feebleminded character in 2e be like outside of combat?" I would say that they would need the assistance of their party to survive at all, otherwise consider the character as dead/lost. Strictly speaking, think 2-yr-old. Or maybe 3.
I would say the most important vector of how the affected character would behave outside of combat is how the player understands what the word "feeblemind" means, so you also get to set the stage and guide them on how you expect their feebleminded character to be played. The end goal is game balance and keeping your group engaged in your story as a DM :) so you have plenty of artistic license in 2e/3e.
Or if you want to be more realistic/playable (though playing a screaming 2-yr-old is fun) you could consider the origins of the phrase "feeblemind" most likely coined to describe things we know of as Advanced Alzheimers or Senile Dementia, or full-blown Schizophrenia, and if you've ever worked with or lived with someone who has these conditions you will know exactly how this spell affects adults ;) Definitely more playable as some who have these conditions are still able to speak, BUT the language skills are effectively lost because communication breaks down with "sane" individuals. And those who have these conditions are ultimately unable to take care of or fend for themselves.
I've lived with a family member for 50 years who is a paranoid schizophrenic and can say with absolute certainty that they could be called "feebleminded" because their reality is no longer the same as ours. I've also taken care of Alzheimer's patients at many stages of this disease and again, "feebleminded" easily can describe their mental states and behaviors. Any of these conditions effectively disables a person mentally, which is the goal of this spell.
One could say that the best example of a FUN and playable character hit by feeblemind is Ho'dor from GoT. Did He did whatever he was told, only knew one word, and had rudimentary survival skills but needed help in many ways. He was helpful to the group and could "tag along" but wasn't versatile in combat. We would want our beloved player's character hit by a feeblemind to have some hope of salvation, yet still be able to play and I think Ho'dor bridges that gap nicely. Alzehiemer's and Schizophrenia are very difficult to play"play" ;)