The last three companies I've worked at had the same rule, except with no exceptions. All discussions get recorded into the system, precisely for the reason that you state -- so there's a historical record of what was discussed, what was decided, and why.
Any "external" communications, such as with email or IM, are expected to be copy/pasted into the system so its permanently recorded in a useful way.
In practice, these records are only read from beginning to end when someone new to the issue at hand needs to come up to speed. Otherwise, the value is in being able to search through the record to find specific things.
This drives me crazy. Even worse is when I get told out of the blue that something I did is wrong, by the supervisor who told me to do it that exact way some months ago.
This has happened almost everywhere I have worked, and none of those places kept change discussions to a specific ticket. I'm going to push for this approach more in the future, it sounds like the right solution.
As we scaled, we had to transition to a ticketing system, for the reasons you mentioned.
Slack definitely feels backwards in some ways especially if you use it as a channel for dealing with customer requests, especially if multiple people at customer site are part of the slack group.
It's been hard adapting to the... less structured environment I'm currently in, but this is the thing that really gets me.
A company is very unlikely to ever side with a subordinate over their manager.
If he had his status changed from fired with cause to without cause, it would have made a difference in his unemployment. I do not believe that they could deny Cobra. Why would they? He would have to pay the full cost with no subsidy from the employer.
I'm just choosing to believe I'm doing a poor job explaining the tradeoffs, and then time or someone else does a better job a few months later.
This is how email or ticketing systems to an extent work. Though we have a rule that certain ticket discussions should go into a ticket itself so decision changes are not lost and people have accountability (which is better cause I get asked sometimes why I wrote code a certain way by the very person who suggested the change).