Few of the oldest toilets before the water saving mandates started had a jet. THey just dumped lots of water into the bowl until the pressure from that started the siphon. Most modern ones do use a siphon jet to get the siphon started since there is no longer a lot of water height and therefore weight to get it started with any power. One side effect with the newer ones is that there is less water washing the sides of the bowl to remove 'skid marks', if they happen to occur.
Sorry to dispute you, but that's not accurate. Before the 1.6 GPF mandate, only the less desirable toilets had no siphon jet. Apparently, the plumbing manufacturers didn't think the 1.6 GPF mandate would fly, and they definitely did not have suitable product ready for the market. The first 1.6 GPFs, even from the "big boys," had reduced trapways, lower bowl water levels and no siphon jet. The first generation of 1.6 GPF Kohler Wellworth had a small hole in the bottom of the bowl, but I don't know if it was a siphon jet. Of what was available at the time, that Wellworth was the best performer, and that's saying very little. After a few years, according to Lovelore (and others), Toto was the first to re-engineer low flow and the others have followed suit. However it came about, I'm happy that they finally figured it out.
If by "oldest," you were referring to early 20th century and before, then your info was accurate and I apologize for my misinterpretation.