As there seems to be inconsistencies throughout Moodle when using icons for sorting purposes. In 3.7 during the forum project (MDL-65071) it was decided by the UX team to introduce new icons for sorting purposes.

https://fontawesome.com/icons/long-arrow-down?style=regular

https://fontawesome.com/icons/long-arrow-up?style=regular

Ideally, we should replace old icons with the new ones and continue using them in the future to have more consistency.

For more info see the Forum's discussions list page (mod/forum/view.php?id=XX)