I've been using this concept to coach my mentees lately but I don't call it "I-shaped marketer", I call it a full-stack marketer.
For example, within paid/performance marketing, I coach my mentees to learn:
• Creative writing (to become good at writing ads)
• CRO (to optimize last mile performance on LP)
• Analytics (to become good at analysing results)
• Automation (to automate routine tasks)
• Research (to understand larger patterns in sales conversations)
Knowing all these skills will help one do their performance marketing job really well and not depend on others as much.