As initially reported issue in ZBX-21009 has evolved, it turned out that adding backslash escaping to the function parameters of history functions would break:

• triggers that have backslashes in history functions parameters
• calculated items that have backslashes in history functions parameters

As such it was decided to roll back changes made in ZBX-21536 and /ZBX-21534 on all branches except on master.

It was decided, going forward, that Zabbix should have proper backslash escaping in parameters of history functions.