Ok, I tested the plugin with PHP 8.2 and the error message is
Fatal error: Array and string offset access syntax with curly braces is no longer supported
So try replacing
$strreplaced = $strreplaced . "&#" . ord($str{$i}) . ";";
with
$strreplaced = $strreplaced . "&#" . ord($str[$i]) . ";";
or maybe use a different extra like emo or ObfuscateEmail-Revo.
Honestly, I think this plugin doesn’t do anything, as $modx->documentOutput is always null when I test it.
Do the email-addresses on your page even get converted?
Maybe use this as the last line instead:
$modx->resource->_output = preg_replace_callback("#<a[^>]*mailto:([^'\" ]*)['\" ]>([^<]*)</a>#i", 'emailaddress', $modx->resource->_output);