It's generally recommended to use IAM role based access instead of AWS access keys.
It would be great if this plugin could support it.
Even if you define an IAM role, you still need to associate this with an IAM user, or otherwise get an access key and secret via STS, right? So in the end, the plugin would still be using an access key and secret; they'll just be short-lived — depending on how you called STS — and stop working at some point.
Or what do you envision being implemented in this plugin?
EC2 Spot Fleet supports IAM Roles:
EC2 Fleet Plugin supports IAM Role. Manage Jenkins > System Configuration, find Spot Fleet Configuration, if you open AWS Credentials, at the bottom, it has IAM Role Support, which allows the specifying role to assume by the plugin.
Feel free to reopen if you have questions.