It's (only) a warning, either disable displaying deprecated and warning messages (because it effectively may break functionality of a page even if nothing else is broken), in local config

$conf['show_php_errors_on_frontend'] = false;

or fine tune message classes to display, see [Forum, post 183126 by erAck in topic 31852] Menubar tags issue after 12.2.0 upgrade, or better, log to file instead.
See [Forum, post 180778 by erAck in topic 31258] Error Message.

To answer my own question,

on unraid
yourhost\appdata\piwigo\www\local\config
edit
config.inc.php

as erAck helpfully said
$conf['show_php_errors_on_frontend'] = false;