Some redundant departments (HR, finance, accounting and the like) will be downsized after the acquisition.
Engineering and product will be unaffected in the short term, but in a year or two the IBM culture will start to seep in, and that would be a good time for tenured employees to start planning their exits. That's also when lock-up agreements will expire and the existing leadership of Confluent will depart and be replaced by IBM execs.
IBM is a really big and diverse company, in a way fundamentally different from most other big tech. In a sense, it is completely incoherent to refer to them as a singular entity.
My opinions are my own. I worked at IBM like a decade ago in a role where I could see the radically different motivations of divisions.
IBM will likely give Confluent employees a large pay package, and then let them go after the merger.
edit: btw, it's typical for any acquisition/merger