The assignment upgrade helper was available in Moodle core from Moodle 2.3 (released in 2012) up until Moodle 3.6 (2018) - in most cases, 6 years of availability should have been enough time for anyone who needed it to run it ...
I would guess that if you downloaded a copy of Moodle 3.5, then copied across the admin/tool/assignmentupgrade folder to your Moodle 3.6 site, then it will probably work fine (
https://docs.moodle.org/36/en/Assignment_upgrade_tool - has a link to the Moodle tracker ticket where the removal was done, that explains that most of the functions used by the tool are still there, to support restoring from old backup files).
