wp_load.php “Hook custom_css_loaded is deprecated since version jetpack-13.5!”

I have a PHP application which is authenticating users against WordPress by including wp_load.php and then using wp_authenticate() etc. In the last couple of days I have started getting the following errors every time the application includes wp_load.php:

E_USER_DEPRECATED in /home/****/public_html/wp-includes/functions.php at line 6078
Hook custom_css_loaded is deprecated since version jetpack-13.5! Use WordPress Custom CSS instead. Jetpack no longer supports Custom CSS. Read the www.ads-software.com documentation to learn how to apply custom styles to your site: https://www.ads-software.com/documentation/article/styles-overview/#applying-custom-css
#0 MRBS\generate_backtrace() called at [/home/****/public_html/book/functions_error.inc:228]
#1 MRBS\output_error() called at [/home/****/public_html/book/functions_error.inc:274]
#2 MRBS\error_handler()
#3 call_user_func_array() called at [/home/****/public_html/wp-content/plugins/wpforms-lite/src/ErrorHandler.php:169]
#4 WPForms\ErrorHandler->error_handler()
#5 trigger_error() called at [/home/****/public_html/wp-includes/functions.php:6078]
#6 wp_trigger_error() called at [/home/****/public_html/wp-includes/functions.php:5941]
#7 _deprecated_hook() called at [/home/****/public_html/wp-content/plugins/jetpack/modules/custom-css.php:35]
#8 custom_css_loaded() called at [/home/****/public_html/wp-includes/class-wp-hook.php:324]
#9 WP_Hook->apply_filters() called at [/home/****/public_html/wp-includes/class-wp-hook.php:348]
#10 WP_Hook->do_action() called at [/home/****/public_html/wp-includes/plugin.php:517]
#11 do_action() called at [/home/****/public_html/wp-content/plugins/jetpack/class.jetpack.php:1854]
#12 Jetpack::load_modules() called at [/home/****/public_html/wp-includes/class-wp-hook.php:324]
#13 WP_Hook->apply_filters() called at [/home/****/public_html/wp-includes/class-wp-hook.php:348]
#14 WP_Hook->do_action() called at [/home/****/public_html/wp-includes/plugin.php:517]
#15 do_action() called at [/home/****/public_html/wp-settings.php:550]
#16 require_once(/home/****/public_html/wp-settings.php) called at [/home/****/public_html/wp-config.php:66]
#17 require_once(/home/****/public_html/wp-config.php) called at [/home/****/public_html/wp-load.php:50]

Any ideas?