io/ioutil package, like most things with util in the name, has turned out to be a poorly defined and complex to understand the collection of things.

In a series of a few changes, the entire ioutil package will become deprecated starting from Go 1.16.

Existing code using ioutil will continue to work; ioutil will consist of simple wrappers to new functions which reside in the io and os packages.

Bad practice

ioutil.ReadAll(r)
ioutil.ReadFile(filename)
ioutil.ReadDir(dirname)
// ...

Recommended

io.ReadAll(r)
os.ReadFile(filename)
os.ReadDir(dirname)
// ...

References

• Go issue #42026
• Go issue #40025