String parameters as well as string inputs, outputs are valid variables defined in the FMI standard. You can find e.g. an example in the Reference FMUs here: https://github.com/modelica/Reference-FMUs/tree/main/Feedthrough

Some simulation tools do not support the String variables at all as they are only intended for numerical simulation with numerical data types. So they are not fully FMI compliant.

However, I would say that most tools support string parameters, while string inputs/outputs are only supported by a minority of tools.

In FMI 3.0 supporting tools the situation is better, as most of them support binary variables, a concept that is not far away from string variables.