Comment # 10 on bug 1195391 from Dirk Mueller

(In reply to Stanislav Brabec from comment #9)
> But maybe it is supported in all shells that can provide /bin/sh.

Its fine to depend on bash for now, don't worry about it. if it becomes a
problem we can look at making the script posix sh compatible. 

> Is there any document, which shell features can be used with #!/bin/sh?

Well, the answer is the POSIX:2008 or SUS specification ;)

I find

https://mywiki.wooledge.org/Bashism

a good reference as well as the "checkbashism" utility.