That's not exactly true. The d20 System Trademark License was a thing layered on top of the OGL.WotC is free to re-release the same material under a different license. But, that new license is merely another option that may be used, not must be used. They cannot prevent anyone from using the older one, because WotC cannot control whether a licensee got a version of the text with the new or old license.
And, in fact, WotC did this - back in the day, 3e was licensed under the OGL, or the d20 license, depending on whether the publisher wanted ot use the d20 logo and actively claim compatibility with D&D or not.
The OGL has a clause that says, paraphrased, "You may not use any other party's trademark in conjunction with a work containing Open Gaming Content, including as an indicator of compatibility, without a separate license covering the use of the trademark." This means that you're not allowed to say "An adventure for Dungeons & Dragons" or "Requires the use of the Dungeons & Dragons Player's Handbook" or things like that. The d20 System Trademark License was that separate license, saying "You get to use the d20 System Logo and say that your stuff is for the d20 System, and refer to the core books in a limited fashion, but in exchange you can't make your thing into a separate game, and you can't change certain definitions."
The d20 STL was not irrevocable, and was changed multiple times (most famously to block the Book of Erotic Fantasy) and eventually revoked. That's why you today see things like "For use with the 5th edition of the world's most popular role-playing game".