I may be missing something, but neither the warning nor the TypeError look legit to me.
The function should just return false instead IIUC.

It appears to be deliberate in the sense that the problem was classified as a sort of "type error" and was thus updated to throw actual TypeErrors.

Quickly scanning over the source, the logic for whether that exception is thrown is also used with whether stream_isatty() returns true. Thus I would suggest a change like

  if (\DIRECTORY_SEPARATOR === '\\') {
      return (\function_exists('sapi_windows_vt100_support')
+         && stream_isatty($this->stream)
-         && @sapi_windows_vt100_support($this->stream))
+         && sapi_windows_vt100_support($this->stream))
          || false !== getenv('ANSICON')
          || 'ON' === getenv('ConEmuANSI')
          || 'xterm' === getenv('TERM');
  }

Before I found stream_isatty() I would have agreed that a warning is more appropriate, but now I'm not so sure.

Either way, I think the docs for sapi_windows_vt100_support() should mention stream_isatty() too.

Ugh.  First, it should be noted that the function throws a
TypeError instead of a warning with PHP 7 if strict_types are
enabled.  That suggest that the function was intended to only be
called on valid "ttys", so I tend to change this ticket to doc
bug.  I'm not really sure about this, so further input would be
welcome.

I think I agree with @nicolasgrekas here that this should just return false (without warning) for non-consoles (in fact it already does for some of them -- but only those that are castable to FDs).

Not even emitting a warning when used as getter is fine for me as
well, but I think consequently when used as setter, the function
shouldn't throw (but raise a warning only).