This can be seen e.g. on gentoo-portage-dev mailing list: https://archives.gentoo.org/gentoo-portage-dev/threads/2016-05/ All mails sent using git-send-email are treated as separate threads, as well as replies to them.
It seems to work properly for the gentoo-dev list. For example: https://archives.gentoo.org/gentoo-dev/message/465322d3821b7b35192c79f4ee047cec
(In reply to Mike Gilbert from comment #1) > It seems to work properly for the gentoo-dev list. For example: > > https://archives.gentoo.org/gentoo-dev/message/ > 465322d3821b7b35192c79f4ee047cec Threading is sometimes recalculated per-list, you'll see flat messages during that time, affecting all messages, not just g-s-e.