You can try customizing the template file. You can find the default path on your system by typing rmarkdown:::rmarkdown_system_file("rmd/ioslides/default.html") in the console.
The easiest way to play with it is to copy it to your project directory, rename it and add an additional YAML parameter, for example template: custom_template.html . Although this may not be enough to completely remove the title slide, you can always customize it so that it looks like the first slide of the presentation.
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