It depends on what you consider to be speculative. It's certainly not a product available to the mass consumer market, if that's what you mean. But that's also kind of an uninteresting fact, right?

"Synthetic genomes unveil the effects of synonymous recoding" https://www.biorxiv.org/content/10.1101/2024.06.16.599206v1

"Probing the limits of genetic recoding using multi-omics-guided evolution" https://www.nature.com/articles/s41467-026-74300-9

"Escherichia coli with a 57-codon genetic code" https://www.science.org/doi/abs/10.1126/science.ady4368

Speculative whether it could be done with humans without severe side effects that would make it useless/pointless.

Is it a product available to humans at all?