The lab I used a decent bit in Kentucky between 2015 and 2020 had a minilab-type machine for E6. In fact it didn't look all that different from the C41 Frontier sitting next to it other than being older and somewhat larger/longer.
The last E6 I sent them they ended up returning undeveloped as their machine was down. The last I spoke to them, they still expected to have it back online but it had been a drawn-out process. This lab apparently has/had mail order contracts with several camera stores and smaller labs in a several state radius, and that kept enough volume flowing in to run it a few times a week. I was one of the few people walking in the door with E6 at the time.
The pro lab I mentioned using(that closed in 2007) did use a huge spliced-roll continuous processor for E6. They did C41 on D&D.