I record them in a different system that is more likely to survive shifts in technology, business relationships, and arbitrary expiration date changes to save money.
What is this other system? Adding to a ticket is alright but who does it in the team? Most of the engineers they don't do it and PMs are generally not aware of half the discussions which are tech related.
Some slack threads are automatically mirrored as c comment threads in our ticketing system.
For others, I add "for future reference" comments with quotes from Slack.