Speaking from the other side (the side that does the termination), as long as your IT team is actually good a simple ldap diff isn't going to be enough.
Why? Because a good termination process is sensitive to there needing to be a communication about a termination that can happen well after the actual process of eliminating their access and telling them it's their last day.
So a better termination process is something like:
1. Employee goes to a physical space (preferred) where they don't have their work equipment or talk to their manager and/or HR using something that isn't work controlled (phone call, etc.).
2. A manual or scripted process executes that forces sign outs of all work things (computer, slack, google, whatever). Credentials get reset and not disabled. Perhaps someone can try to look for password reset metadata or other things that might indicate a departure, but it's a lot harder than looking for disabled uids.
3. After the person leaves or has finished their conversation remotely, the team that works with this person gets a broader communication from someone to tell them about the departure. If the company is small enough, maybe there's a broader communication to more people.
4. The rest of the termination process gets fired off that does disable accounts, etc.
Why don't all IT departments do this? Well for a lot of reasons:
1. They don't care, don't have incentives, or haven't been told by HR, etc. to care about handling the termination process in a more sensitive way.
2. For any sufficiently complex company, the number of edges cases of systems where you can't force a logout or handle a password reset increase over time. It takes a lot of testing to make sure a process works because vendors have bugs all the time or unintended behavior.
3. The risk of poorly communicated terminations increase as the number of people that either perform or can troubleshoot the automated process to terminate increase. As others commented, you don't want some ticketing system that is readable by a wide amount of people to see termination requests, so now how do you communicate a termination without too many people knowing about it?
Strangely enough, I think trying to achieve the most sensitive but automated process is good because it forces the company to communicate and acknowledge a departure before the full termination process fires off, but maybe I'm in the minority.