Is it common/ ethically acceptable for PhD advisors to propose and supervise topics on which they lack basic knowledge?

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 "acceptable" 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.

I would usually expect that the a supervisor had some connection either to the application domain, or the technical domain of a project.

I'll give examples from biology, because that's what I do: A PhD might be to use a particular technique, say a new type of microscopy, to study a particular biological process, say cell division. I would expect a supervisor to be an expert either in cell division, or in microscopy (but perhaps not both).

The best situation would be to have two supervisors, one who was an expert in microscopy and one who was an expert in cell division, but thats not always possible.

I think if your supervisors were unable to give you any guidance, on any aspect of your project, right from the start, then they probably shouldn't have been supervising you. Or at least, not on that project.

The existing answers give a good perspective on this situation, but I don't think they directly address OP's question:

How "normal" 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 STEM?

In short: not common. Some cases when I've seen this happen:

• A project deviates from the initial concept because a student insists on a particular approach
• A colleague leaves the academia, and somebody needs to take over the supervision of a student
• The supervisor might want to branch out to another field out of personal interest
• There is a new hot topic in the supervisor's field, and having some work on that may improve the group's chances to get funding
• There is a "low-hanging fruit" project which is likely to produce easy publications or help secure funding, but it is not scientifically interesting to the supervisor

Generally speaking, working outside one's primary field usually means less immediate returns for the supervisor (in terms of funding and publications), so in my experience groups which are primarily funded through grants tend to specialize quite narrowly to keep the machine running.

On the other hand, taking more students may be incentivized by local institution rules - the number of total active students, or students with external funding, may be a metric used to judge the performance on the department/group. I think this is most likely what happened here, given your description. Either way, good job on persisting, and don't spend too much time trying to guess somebody's reasons for whatever they did.