I've looked through this website for answers but most of the questions pertaining to the spell are for 5e, like this one, where the spell has gone through many generations of changes. It doesn't really help that the 2e version of the spell itself doesn't really provide much about mechanical or roleplay effects on characters either. It merely states this:
The feeblemind causes the subject's intellect to degenerate to that of a moronic child. The subject remains in this state until a heal or wish spell is used to cancel the effects. Magic-using beings are very vulnerable to this spell;
Whereas the 5e version is completely different:
You blast the mind of a creature that you can see within range, attempting to shatter its intellect and personality. The target takes 4d6 psychic damage and must make an Intelligence saving throw.
On a failed save, the creature's Intelligence and Charisma scores become 1. The creature can't cast Spells, activate Magic Items, understand language, or communicate in any intelligible way. The creature can, however, identify its friends, follow them, and even protect them.
At the end of every 30 days, the creature can repeat its saving throw against this spell. If it succeeds on its saving throw, the spell ends.
The first major change, aside from including proper mechanical effects, seems to be that the 5e version attacks intellect and personality whereas the 2e version attacks only intellect. The next is that 5e's feeblemind does psychic damage whereas 2e's doesn't. 5e's is also more lenient, giving monthly opportunities to save again and adding a third spell that can remove the effect, compared to 2e where the effect is simply permanent and only heal or wish can remove it.
Looking at the spell in other editions, it consistently reduces Intelligence and Wisdom to 1 as the mechanical effect. This is similar to the mechanical effects of the 5e version of the spell which reduces Intelligence and Charisma to 1, with Wisdom being substituted for Charisma in the rule as Intelligence and Wisdom are the two spellcasting stats in 2e.
However, we're finding some difficulty in translating the mechanical to roleplay and looking at 5e-based answers doesn't help much. The stats function differently enough that 2e needs its own answer. For example, language is tied to Intelligence in 2e whereas this seems to be saying that language has nothing to do with Intelligence in 5e. In the 2e Player's Handbook Intelligence is described as
Intelligence (Int) represents a character's memory, reasoning, and learning ability, including areas outside those measured by the written word. Intelligence dictates the number of languages a character can learn. Intelligence is the prime requisite of wizards, who must have keen minds to understand and memorize magical spells.
This ability gives only a general indication of a character's mental acuity. A semi-intelligent character (Int 3 or 4) can speak (with difficulty) and is apt to react instinctively and impulsively. He is not hopeless as a player character (PC), but playing such a character correctly is not easy.
which helps give an idea of how a character with low Intelligence behaves but only really provides a vague notion of how a feebleminded character might compare. An Intelligence of 1 means a character cannot speak any languages but "can communicate with grunts and gestures" according to the ability scores table. The same table also tells us that a feebleminded wizard (Int 1) can cast no spells per day.
The section on Wisdom describes the stat as follows:
Wisdom (Wis) describes a composite of the character's enlightenment, judgment, guile, willpower, common sense, and intuition. It can affect the character's resistance to magical attack. It is the prime requisite of priests;
and the table dictates that a Wisdom of 1 results in an 80% chance of spell failure, but the entire section doesn't provide examples of behavior for low or high scores.
So, we know that casting and anything but the most basic communication are essentially impossible, making a caster pretty much useless in combat. That would be all well and good except there's a lot more to a campaign than combat. In fact, those campaigns which I run or participate in are mostly roleplaying with a little combat here or there. So, what would a feebleminded character in 2e be like outside of combat? I don't really know what constitutes a "moronic child" as the spell's description states, though the use of "moronic" seems to imply they would be even less capable than your typical young child. Could they even take care of themselves or would they need assistance with basic survival needs like feeding themselves? Some might jump to "they would default to behaving on instinct like an animal" but human beings don't typically fall back on instincts in the same way animals do in situations where intelligence is lacking or impaired so I doubt that would be very accurate.