My shared hosting service has forced me to use PHP 7.2 as of today, so I needed to suppress the deprecated warnings from the error logs as described above. I used Chen Zhen's solution with improved version comparing, so it looks like this:
// Disable PHP 7.2 "Function create_function() is deprecated" errors from filling the forum error logs
if (defined('E_DEPRECATED') && $error_level == E_DEPRECATED && version_compare(phpversion(), '7.2', '>=') && strpos($error_string, ' create_function() is deprecated') !== false)
return;
However this did not seem to suppress all warnings, only about half of them. It seems that some are already generated before the error handler is set in index.php (I think particularly the reloadSettings() function is causing them, but I am not very familiar with the core functions of SMF). So I cut the following code from index.php...
// Register an error handler.
set_error_handler('error_handler');
...and pasted it a bit higher, right below this line:
require_once($sourcedir . '/Errors.php');
I thought this might be useful to anyone else having these issues. So far my forum seems to be running perfectly fine, hope it stays that way while I keep my eager eyes on the progress made on SMF 2.1 release candidates.

If anyone thinks this is a bad idea, please let me know.