Reddit made it impossible to have long-term discussions like forums do. Posts would just fade into irrelevance after a day or so, whereas with forums new comments would bump the thread.

Lemmy has a sorting option just like forums:

"New Comments: Bumps posts to the top when they receive a new reply analogous to the sorting of traditional forums"

And Lemmy also has the "Active" sorting method, which says:

"Calculates a rank based on the score and time of the latest comment, with decay over time"

so Active seems like a compromise between Hot like Reddit and the way forums do it by New Comments

https://join-lemmy.org/docs/en/users/03-votes-and-ranking.html