I can't find it, but I seem to remember there were plans to change the behavior of .on1 sidecar files for On1 Photo RAW. If that has occurred, it's not happening on my system.
With View --> Unknown Files as Proxies turned on, for those of us using On1 Photo RAW, the contact sheet is cluttered with .on1 files. Turning off this option removes the clutter, but we are then unable to see proxies for .on1photo, .afphoto, .pld or other native files created by third-party editors not supported.
Sidecars for DxO-edited raw files are paired with the files. They are not revealed in the contact sheet, but deleting the DxO-edited file also deletes its sidecar. The behavior for .on1 sidecars should be the same as those for DxO.
macOS Mojave Version 10.14.5
iMac (Retina 5K, 27-inch, 2017)
Procesor 4.2 GHz Intel Core i7
Memory 32 GB 2400 MHz DDR4
Graphics Radeon Pro 580 8 GB