1. My biggest issue with those questions is not product recommendations, but lack of effort.
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["How to ask a good question"](https://superuser.com/help/how-to-ask) is quite clear on it:  

> Have you thoroughly searched for an answer before asking your question? Sharing your research helps everyone. Tell us what you found and why it didn’t meet your needs. This demonstrates that you’ve taken the time to try to help yourself, it saves us from reiterating obvious answers

I know sometimes even phrasing the question well is already difficult, however in this case anybody finds superuser and can type a question should also be able to google for "printer xy driver OS z".

- I think majority of OPs requesting driver just don't take the effort to look for it
- other case can be that OP finds too many pages and doesn't have the time to read through them. Asking for the driver this case means OP expect us to do their job, clearly off-topic here.
- sometimes - as you mentioned too - there is no driver for the specific OS, multiple version ... If the OP has already searched then all these information can be included in the question, and the question won't be off-topic anymore.  Instead it'll be a well-written, on-topic question.

2. link-only
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Link only answers are not preferred here and for good reasons. For driver recommendations we can only give link only answers, so the site can't even contain the answer (driver itself)

3. product recommendation
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What should you do?
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- When I see an off-topic question my first action is to vote for closing it.  
In my opinion the quicker a question is closed the sooner low-quality answers are prevented and its author is warned that question has an issue. Also close explanation always contains a link to help center, so OP can learn how to improve.
- you can always write a comment suggesting to google for it (or just a link to this question:) ). Anyway if really keen to find the answer then OP can also write a comment asking for further support and we can help finding the direction.

> Maybe we could create a custom close flag for this (where, in the description, will be given a quick explanation on how to search for drivers).

Unfortunately there are a lot of types of off-topic questions which occurs frequently, there is just no place to add a custom flag for all of them. 

Also, I don't think one user would ask driver recommendation several times, they are different users asking for it once. As they don't google for the driver itself, they will also not browse previous driver recommendation questions. So I don't think having a custom close flag would help to decrease the number of such questions.