It's actually more accurate to say that it used to be that single-letter TLDs could be registered, however

> Only three of the 26 possible single-letter domains have ever been registered under the .com domain, all before 1992. The other 23 single-letter .com domain names were registered January 1, 1992 by Jon Postel, with the intention to avoid a single company commercially controlling a letter of the alphabet.

and

> On December 1, 1993, the Internet Assigned Numbers Authority (IANA) explicitly reserved the remaining single-letter and single-digit domain names. The few domains that were already assigned were grandfathered in and continued to exist.

https://en.wikipedia.org/wiki/Single-letter_second-level_dom...