Why can we not ask for the best practice of code?

Question

Good code reviews are the bar that all of us should strive for. They cover common and easy to follow best practices that any team can get started with, while ensuring high-quality and helpful reviews for the long term.

code review says about clean code (readability, easy to understand,care about names,...) and performant code (Levels of optimization, Design level, When to optimize, Trade-offs, Strength reduction, Run time, Assembly level, Compile level, Build level, Source code level,...)
— How to Make Good Code Reviews Better, Gergely Orosz

When I asked about performance of some code or ask about best practice of some code the community said to me Code Review is not about asking for how to implementation best practices.

As you can see above, the community said to me that generic best practices are outside the scope of this site! Why then is the name of this site Code Review? When one items of a code review is about best practice of code.

Answer 1

Note, your question in this meta post is about why asking about best-practices in general are off topic, and you provide some example questions which were closed.

Note, those questions were not (necessarily) closed because they were about best practices in general. The close reason covers a number of different possible reasons for the closure. The first linked question (Date convert) for example, has no context describing what the conversion is, or should do. It's hard to review code if you don't know the purpose of the code. The "best practices" in the close banner is superfluous to the actual reason, and is only there because it's boilerplate text that covers a number of close reasons.

So, you cannot use one small part of the close-banner to cover the entire close-reason for the question.

Now, as for the general question about why best-practice as a general topic is off-topic, it's for a few reasons:

1. It's highly opinionated, and dynamic. It's one thing to have a question "Does this code follow best practices?", and quite another to say "what are the best practices?"
2. There's the Software Engineering site where such questions are on-topic, and specifically on topic there.
3. This is Code Review, and we review code.

In real life, when doing a code review, there is a lot of lattitude for various discussions, what the best practices may be is part of that. But, on this site in particular, it's all about the real code, as presented.