Considering all available evidence, I think it's very reasonable to assume that the democrats filled out the official subpoena forms and included them with all of the subpoena letters they sent out in regards to the impeachment inquiry, even before October 31.
I understand that the only evidence that you are willing to consider is whether or not you are able to see a picture of the actual subpoena, and think it's reasonable to assume that the democrats didn't fill out the official subpoena forms and include them with the subpoena letters.
You mean some people talked about it? People talk about lots of things that don't exist, the Trump Ukraine quid pro quo for example. Your assumptions are not a standard of evidence, neither are the words of others.
Actually the words of others are absolutely a 'standard of evidence'. In fact, believe it or not, subpoenas are often used to get people to say words that are then used to as evidence. It's true that people talk about lots of things that don't exist, so you should consider the credibility of the person speaking as well as what they are saying.
If, for example, the Secretary of State says he received a subpoena, and he had no motive to lie about receiving that subpoena, then it should definitely be taken into consideration when considering whether or not the subpoena actually exists.