Not a constant list, because it is not a property of this library: it is a property of the machine. Two engines contribute, and they do not cover the same formats.

• PharData compresses through PHP extensions, so it offers gzip with ext-zlib and bzip2 with ext-bz2, and nothing else — Phar knows no other.
• The system tar pipes through a program, so it offers whatever it can find: gzip, bzip2, xz and zstd. Which means a host can write .tar.zst and the one next to it cannot, with the same code and the same version of this library.

none is always there.

Why this is a function and not documentation. tar() used to refuse xz with "Compression type 'xz' is not supported", raised by a helper assembling a temporary path, seventeen lines before the engines had even been looked at — a sentence about this library where the truth was about the host. The refusal now names what is missing and what would work here, and this is where it reads it. It is also what a doctor command should print: a host that silently writes .tar.gz where the operator expected .tar.zst has nothing else to tell them.

Deliberately absent: lzma, which xz superseded and whose --lzma flag tarCompressionFlag() still carries for a caller going straight to tarWithBinary(), and lz4, for which GNU tar has no flag at all.