My original understanding was that a Mainframe is a big 'bucket-like' centralised computer with a few dumb terminal(s) and without comms. In Traveller, dumb terminals are TL 7, and comms are TL 8+. In the real-world I'm reasonably sure 'Mainframe' gets replaced by comms-friendly distributed architectures, such as client-server, or data centre, or cloud-based edge or fog computers. In parallel architectures, they get replaced by computing clusters or supercomputers. And so on. In fiction, you want ship's brains, etc, at higher tech levels. So I suspect the definition of 'Mainframe' became an all but obsolete architectural definition, beyond TL 7, because innovation into comms and/or parallelism made the 'big bucket' idea unfeasible.