When people construct their own tar.Header, it is common to see something like:

tw.WriteHeader(&tar.Header{
	Name: "VERSION",
	Mode: 0644,
	Size: int64(len(version)),
}

This does not set Typeflag: tar.TypeReg and so uses the zero value of Typeflag, which is TypeRegA.
The relevant specification says:

0 represents a regular file. For backwards-compatibility, a typeflag value of binary zero ( '\0' ) should be recognized as meaning a regular file when extracting files from the archive. Archives written with this version of the archive file format create regular files with a typeflag value of the ISO/IEC 646:1991 standard IRV '0'.

Thus, the zero value should be interpreted by readers as equivalent to TypeReg. Every tar reader I know of properly does this. However, it seems like a good idea to have the Writer promote TypeRegA to TypeReg as I can't think of any reason why the user would explicitly want to encode TypeRegA.

Similarly, perhaps the Reader should automatically promote TypeRegA to TypeReg so that users don't have to keep putting both versions in their switch statements.

\cc @bradfitz