This does not appear particularly related to PHP 7.3, see
<https://3v4l.org/CeumU>.

That error appears to be correct in previous versions though. Without the required  line break, the label would be part of the text instead, so it would make sense to expect T_END_HEREDOC.

Yes, in 7.3, the documentation states:
"The closing marker for doc strings is no longer required to be followed by a semicolon or newline."
https://www.php.net/manual/en/migration73.new-features.php#migration73.new-features.core.heredoc

However, if you don't include a semicolon, you get an error:

<?php

$str = <<<FOO
abcdefg
FOO

echo $str;

> Parse error: syntax error, unexpected 'echo' (T_ECHO) in /in/eTKHe on line 7

If you add a semicolon after FOO, everything works. This is against 7.3.3:
https://3v4l.org/eTKHe

> However, if you don't include a semicolon, you get an error:
The semicolon is not required *for the heredoc itself*. You still have to write valid code, and

  $str = "
  abcdefg"

  echo $str;

is not valid code.

Can you provide an example where you would ever not need a semicolon at the end of the heredoc, then?

Yeah. Easily.

echo <<<FOO
abcdefg
FOO, "hijk";

> The semicolon is not required *for the heredoc itself*. You
> still have to write valid code, […]

Right.  Changing to doc problem.