Assuming I want to listen to subliminals with my headphones which have an standard audio jack connection/cable:
Do I still need a cell phone with an audio jack or can I use a USB-C adapter for the connections?
Are there any advantages or reasons to listen via an audio jack?
Technically speaking, one connection is analogue and the other digital.
Does this still play a role in practice? Do you have a recommendation for me?
Thank you! 
AFAIK there’s no difference. At least I didn’t see any conversation on this issue.
IIRC some people also use bluetooth headphones, I did in the past.
The only thing that you shouldn’t use are Noise Canceling Headphones because SC can’t tell what they will cancel from the subs frequencies. So it might lead to your subconscious not getting the actual sub your listening to, but only parts of it, if at all.
Audio jack and a good ol MP3 player. I don’t fully trust yet that Bluetooth or wireless stuff to accurately transmit subliminal messages. I’m not saying they don’t. I’m just saying I don’t trust it. 
Thanks for your answers!
As you already mentioned, I also could not find any conversation about it in the forum.
OK, then I assume that there is no “official” or technology-based recommendation for or against jack or usb-adapter?
Further opinions are very welcome
If it can provide the audio without disruptions, you get the same quality as with audio-jack or USB-C it makes no difference.
If you have a low-quality USB-C adapter, which diminishes the quality of the audio, that might affect the subliminals as well.