According to https://docs.moodle.org/dev/Blocks#db.2Faccess.php blocks that set 'my' to false in applicable_formats() do not need to define myaddinstance. However if the block does not define the capability going to /my and turning on customization causes this to be displayed.

The block spam_deletion does not define the standard capability block/spam_deletion:myaddinstance
line 578 of /blocks/moodleblock.class.php: call to debugging()
line 551 of /blocks/moodleblock.class.php: call to block_base->has_add_block_capability()
line 1124 of /lib/blocklib.php: call to block_base->user_can_addto()
line 1104 of /lib/blocklib.php: call to block_manager->user_can_delete_block()
line 248 of /blocks/moodleblock.class.php: call to block_manager->edit_controls()
line 962 of /lib/blocklib.php: call to block_base->get_content_for_output()
line 1014 of /lib/blocklib.php: call to block_manager->create_block_contents()
line 392 of /lib/outputrenderers.php: call to block_manager->ensure_content_created()
line 45 of /theme/clean/layout/columns3.php: call to core_renderer->standard_head_html()
line 940 of /lib/outputrenderers.php: call to include()
line 864 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
line 167 of /my/index.php: call to core_renderer->header()

This can be reproduced using the spam deletion block. https://github.com/moodlehq/block-spam_deletion.git. Note that MDLSITE-3748 is introducing 'my' = false. If that MDLSITE issue has not been resolved yet you will need to manually make the change.