Sorry, I coded up the ThingSpeak publisher ages ago only to prove to myself that I could get an MQTT publisher working. I’ve never used it beyond that. I don’t have a paid account so after the trial period I was locked out and I’m not willing to do much testing on an account tied to my personal email.
Anyway, the code for ThingSpeak is in the ThingSpeak publisher in the publishers folder. It’s using PubSubClient under the hood. The outgoing message is formatted based on whatever the required structure was at the time. If that’s changed, then the whole formatting step will need to be rewritten.
I wouldn’t guess the restructuring of the json or whatever ThingSpeak wants would be too hard, but I don’t have time to look at it right now. If ThingSpeak now requires a secure connection (SSL, etc) or if they more want more metadata then there might need to be more tweaking for that.