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Most role playing games today have a meta-rule or unified mechanic. In fact, this is considered a hallmark of modern role playing design.
The first might have been the DPG task system for classic Traveller. (Which evolved into the Megatraveller task system.)
While I have nothing against unified mechanics (& they certainly appear in my attempts to design games), I tend to not think of it as much of an innovation.
It all comes down to picking a chance of success & using dice (or other means) to determine success or failure. If you have a 50% chance of success, it doesn’t really matter whether you’re trying to roll 10 or less or d20, 11 or more on d20, or an even number on a d6.
The reason thief skills in old school D&D used d% instead of d20 is because the designer felt they warranted finer than 5% increments.
Also, it should be noted that very few games really manage to have a universal mechanic. e.g. the d20 system has the following mechanics that don’t use the unified mechanic:
(There’s also a few things in the d20 system that use d% instead of d20 for seemingly no reason. There was a reason, though: The designers used this to indicate that they felt modifiers to these rolls should be avoided because it could easily upset the balance they were working so hard to achieve.)