When we rename the class we put in an alias as part of our deprecation policy.
That rename is removed after a period (4 releases) and it was removed in MDL-57244 for Moodle 3.6.0 of Moodle.
It was fixed in the plugin in version 3.5.3 onwards (https://github.com/academic-moodle-cooperation/moodle-mod_organizer/commit/34fc7b3d3db946a18e1a971d395991acf1ded896#diff-352a1931f83c36b7fbe56c2619ab244f).
It may be that Moodle 3.7 introduced a change which meant that the relevant file was included when it wasn't before.
That rename is removed after a period (4 releases) and it was removed in MDL-57244 for Moodle 3.6.0 of Moodle.
It was fixed in the plugin in version 3.5.3 onwards (https://github.com/academic-moodle-cooperation/moodle-mod_organizer/commit/34fc7b3d3db946a18e1a971d395991acf1ded896#diff-352a1931f83c36b7fbe56c2619ab244f).
It may be that Moodle 3.7 introduced a change which meant that the relevant file was included when it wasn't before.