-1 I like the idea but I think it shouldn't be here at the Judaism but rather at https://meta.codidact.com/ No reason Hebrew couldn't be used there too. Please suggest it there!

I think something along the lines of (but definitely a bit different) SE's "this is a new user, be nice" is really all we need. For a new but serious user, it really doesn't take long to get the hang of how to properly ask a question. The non-serious user isn't going to bother with a sandbox - they just jump right in with "Do my homework" or "read my spam". The serious user will figure it out quickly enough if we treat them reasonably well.

That is not to say there isn't a place for Sandbox - if we ever have Code Golf, that absolutely needs a Sandbox, because of the need to get a question nearly perfect before anyone starts Replying. But that is not the case in Judaism or most other Codidact communities.

I like the idea of a sandbox but I think one post should suffice for it. Or one question and its answers and comments, let's say. So it can be in our meta category.

"One question and its answers" for Sandbox was the only way to do it on SE. I didn't like it there, and I don't like the idea of doing it that way here. Categories are a much cleaner way to do this on a community where sandbox makes sense.

@manassehkatz re your last comment: I agree that if we're to have a Code Golf SE -type sandbox then it should be a category of posts. (But I don't think we need one.) My "one post should suffice for it" comment was about the kind of sandbox described in part of this answer: one used for trying out formatting, etc.