[mtha]

work fine in 2.3.0

However, there's a small note:
in the install_signaturemaxchars.php file, there's a install db section:

// ###################### Start install db #######################
if ($action=="db") {

  doformheader("$installer","done");
  maketableheader("Modifying your database");
  $DB_site->query("INSERT INTO setting VALUES (NULL,8,'Maximum Characters Of Signature','signaturemaxchars','100','The maximum number of characters that you want to allow in users signature. Set this to 0 to disable it.','',15)");
  maketableheader("Add signaturemaxchars....Success!");
  maketableheader("Adding Template error_maxsignature ");
  $DB_site->query("INSERT INTO template (templateid,templatesetid,title,template) VALUES (NULL,'1','".addslashes(error_maxsignature)."','The signature text that you have entered is too long. Please shorten it to \$signaturemaxchars\ characters long.')");
  maketableheader("Add Template....Success!");

  doformfooter("done","",2);
  #echo "<p><b>Finish $hackname $hackversion installation,".makelinkcode("Next Step ==>","$installer.php?s=$session[sessionhash]&action=done").".</b></p>";

In this case, the templatesetid for error_maxsignature is 1.
If your templatesetid is different, correct it or add into your current templateset.