Ngtjyty

I've only played Dungeon and Dragons in its 5th edition, and only limited exposure of earlier editions from PC games, but never understand the rule behind it (or know the calculations), so I've never noticed whether the term natural 20 and its significance, including critical hits and success, is applied in earlier editions.

I've watched and read some DnD-themed comics and clips, and I've noticed there are plenty examples that suggest that rolling natural 20 means critical success, i.e: succeeding in whatever you are attempting, regardless how stupid and bizarre the result may be, including seducing the BBEG and marrying them.

I'm mainly interested how and when rolling natural 20 is introduced as critical success. This may be in attack roll, ability check, or saving throw. The earlier the better.

The first appearance of natural 20 as a critical hit in published rules seems to be in AD&D2e, on p. 86 of the Dungeon Master's Guide. It's explicitly an optional rule. AD&D1e specifically disclaims the idea, on p. 61 of its Dungeon Master's Guide:

Such rules as double damage and critical hits must cut both
ways — in which case the life expectancy of player characters will be
shortened considerably — or the monsters are being grossly misrepresented
and unfairly treated by the system.

Natural 20s as critical hits were certainly in use as house rules long before AD&D2e. When I started playing in 1979, one of the groups I played with were running a modified version of Original D&D, deliberately avoiding most AD&D innovations, and they had been using natural 20s for years.

Empire of the Petal Throne had a critical hit mechanic in 1975. There weren't any articles in Strategic Review magazine about criticals, but there was one in Dragon #39, and there may have been others.

There are references to natural 20s and 1s in the first edition DMG.

The rules for saving throws specify that a natural 1 always fails, and a natural 20 always succeeds. Also in the attack tables natural 20s are called out as special at some point in the progression, there is an option (iirc, I need to check on this and provide supporting documentation) that to-hit scores of 21 and better require a natural 20.

But there isn't any concept of extra damage or other benefits for a natural 20, or extra penalties for a natural 1. (It is the extra-damage aspect that the DMG quote is objecting to, of course.)

Second edition has an optional rule for this as described by John's answer.

Third edition is the first occurrence in D&D rules of a non-optional critical hit rule that gives extra damage, afaik.

EPT probably did have a critical hit rule, I can check on this. However, although TSR published it at one point, and there was probably some cross-fertilization going on (Arneson was one of the players in Professor Barker's game) it was never branded as D&D and the rules were significantly different; I am not sure how relevant it is here.