This is exactly what Hilton did in the 2010s. We stood up an ESB (wso2) between the modern front ends and the legacy mainframe backend, and then broke down the mainframe's functions into microservice domains and wrote them. Had the ESB route traffic to the microservices until the mainframe wasn't doing any work anymore and was able to discontinue it.

There was a lot more complexity that that, but that's the gist of it. By the time I left, my team had carved out the bulk of the mainframe's services and had stood up over 50 microservices.