I see there are some alerting options for RDS, but freeable memory is not one of them, making it impossible to monitor memory usage. Why is it not there, as this metric is available in Cloudwatch Metrics? Can it be added?
I am surprised that you see a different key from what I see(PFA). It should be "Freeable memory threshold". We follow this terminology and hence preferred the following text.
Would it be possible to make it percentage based so it could be configured the same for all RDS instances with different memory sizes?
In general, a freeable memory requirements may vary for different memory sizes are 10% availability for 8GB(~200 MB) memory is not as same as 10% availability for 32GB(~3.2 GB). 3.2 GB is a manageable amount of memory in case of RDS . So , we preferred bytes. Are you sure that you will be requiring this? We will look into this if required.