Anyone fitting MNIST in 2019 was already outdated by several years at the very least. GPT-2 was 2019! We already had zero-shot classifiers then. In fact, the paper for GPT-3 was literally
"Large Language Models are Zero-Shot Reasoners"
These kinds of zero-shot classifiers were already developed and used in-house for many years. They just weren't commercialized as a separate product, because anyone who could use an LLM proper could build layers around it to fulfill any classification task like this.