Depends on how you game OKRs. Implement auditing Q1. Contact all teams Q2. Have 75% of teams Q3. 90% of teams Q4. Just an example but its all in how you sell it. Big IT understands icebergs, you just have to give a good/accurate roadmap and execute if they agree on it.

You might be correct that a lack of insight on this sort of thing is an organizational problem. My experience indicates that if you can't get a list of dependencies based on a series of e-mails down an org chart, something is wrong. Either someone is managing too many services for the level of tooling you have, or there's a complete breakdown in org comms (which happens more in larger enterprises.)