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 &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.