I wish all software products would do that. Chat applications are like a busy town square where everybody is yelling into the crowd and important information doesn't survive the moment, while a discussion forum will eventually grow into a public library which preserves information and makes it searchable (assuming that search engines have access to the information of course).
Huh? Have you not used the search feature in Slack or Discord? Whether it's a forum or a chat, the data structure is largely the same, especially with Slack/Discord as they have threads.
It's a bit disingenuous to pretend like they're the same thing. Explicit design for threaded discussion encourages a different style of communication than design primarily intended for real-time chat. Even if the information exists somewhere in Discord/Slack, it's often harder to find, harder to link to, provides less context, requires login and access to a specific community, isn't indexed by search engines, etc.