Is it common/ ethically acceptable for PhD advisors to propose and supervise topics on which they lack basic knowledge? I've just graduated in STEM in a small but respected university in Europe. I had two supervisors who hold their PhD in my field.  The aim of my PhD was to utilize a particular chemical compound in the synthesis of various materials. Very little literature is available about the topic and neither of my supervisors had ever published a single paper about it.
I had a tough time during my PhD because I was left figuring out completely on my own how to do research without having a real expert beside me I could learn from. Moreover, I was prevented from asking for help to anyone. Whenever I asked them for help, my supervisors would just throw out some buzzwords or vague explanations to the data I had produced, which were seldom appropriate. Sometimes, I was also given the silent treatment. Back then, I thought that this was the way a PhD is meant to be done. You know &quot;you have to be independent&quot;, &quot;your PhD is only yours&quot;....
However, I eventually realized that my supervisors were not those experts they were supposed to be and came up with the reason behind their &quot;hands-off&quot; attitude. I found out they did not  aware of the physical, chemical, and even toxicological (!) properties of the compound I had been working with for the last three years. They were even oblivious to the appropriate scientific terminology and apparatuses relevant to the research topic: one of them was unaware that the term polymerization was also applicable  in the context of inorganic compounds, not just organic or biological polymers, while the other did not know a (very) common technique you are supposed to learn during the second year of your bachelor's degree. As an &quot;expert&quot; of the topic, he should have been definitely knowledgeable of. I could continue with other examples, but my thread is not about shaming my supervisors. Their knowledge and expertise are under their responsibility, not mine.
After my graduation, I left both my supervisors as I felt I had been deceived and lured to work with them. I do not trust them anymore. Right now, I continue to feel as if I were used, exploited and tricked. Probably, my PhD served only as a +1 in their CV. I do not think they were so interested in the project or in my growth as a researcher. At the same time, I do not regret my time as a PhD student. It was a stimulating intellectual experience: I learnt a lot of new things and I met many interesting people during my journey.
My questions are: how &quot;normal&quot; or common is it for a supervisor to put forward a PhD project without any expertise or even knowing the terminology and techniques relevant to it, especially in the STEM field? Is doing so considered ethical?
IMPORTANT: this post is NOT a vent or a blame game. I am not interested in discussing about red flags or the specific behaviour of my advisors. I am sharing my story just as an example to kindle a meaningful discussion.
What I am actually interested in is your opinion and/or experience regarding this kind of supervision style and its possible impact on the professional life of a freshly graduated PhD.
Clarifications:

They asked me to join them in this project as I was one of the top students of my cohort. I know I should have asked more questions to my supervisors when I first met them. But, you know, I naïvely thought that they were experts. Otherwise, why taking the role as a PhD supervisor?

Switching supervisors is not an option here, moreover I couldn't walk out (or run away, as JeffE would say) because I unfortunately realized the lack of knowledge of my supervisors at the end of my project. If I had dropped out from my PhD, I should have paid a hefty penalty of several thousands quid.

I was fully funded by a public institution. Is this the reason why they were not so invested in the project? They would not have lost any of their money.
I can't really judge whether the OP's treatment was appropriate or not, but there are situations in which it would be and, in those situations the answer to whether this is &quot;acceptable&quot; would be yes. I also think it is somewhat common, but far from universal.
The bottom line is that at some point along the line the student becomes the expert in the research topic and leaves the advisor(s) behind. In some situations that comes earlier than in most. The fresh PhD is the world's foremost expert in whatever the specific topic of the research is.
It is also true that in many fields the process of research is distinct from the actual questions asked. In those fields a person can advise another on that process and check that they follow correct process to the end, even without knowing a lot about the specifics. Research questions that rely heavily on statistics fall into such a format, though there are many variations that need to be considered. In mathematics, the validity of a proof can be checked by someone who didn't develop it.
However, for a meaningful and worthwhile advisor-advisee relationship to develop, the advisor(s) needs to be honest about what sorts of help they can and cannot provide. That understanding needs to be reached at the beginning of the relationship and if not satisfactory, then either the student or professor(s) needs to withdraw. Some students (a fair number, but not universal) can excel in situations of minimal specific help. Others (almost certainly the majority) need a bit more. Some need a lot more.
Moreover, a lot depends on what the advisor(s) are willing and able to do to advance the career of an independent student after they complete their work. If they are completely supportive, then there is little reason to be alarmed. The fact that you are exhausted at the completion of your degree is not a valid measure here. Nearly everyone (I suspect) feels that way to some degree.
Red flags arise when you aren't told you won't be helped, or if you are told you will be but abandoned. Other red flags appear when the advisor abandons you after you finish. But having to do much of the work of it on your own, isn't, in itself, a red flag, though it doesn't work for everyone. Be glad you finished successfully. Congratulations.
I'd suggest you reevaluate your decision to leave them behind. It may not be the right decision.