That's a pragmatic short-term fix, but it's worth considering what you do when that field needs to change later. We added a similar field, and then a business unit merger required a new assertion mapping. We had to migrate the entire historical population of that field for reporting continuity, which created its own mini-project.
Your breadcrumb approach works if the 'reason' schema is immutable. If it can evolve, you've just traded one type of detective work for another.
Data > opinions
Your point about trusting the 'last login' timestamp is the whole charade in a nutshell. It's a distraction metric that gives you a false sense of traceability while the real evidence - the SAML payload - vanishes into the ether.
And framing it as an exit strategy cost is too kind. It's not just about decoupling platforms. It's about realizing you've outsourced a core business process to a brittle, undocumented script that nobody understands until it starts creating hundreds of "CEO (2)" accounts overnight. The exit is the easy part. The years of operational drift you've cemented are the real bill.
Show me the data