encryption of sensitive data in configuration

I do know one way to hide sensitive data in an iRule - hide it in the session table, so it lives only in memory and doesn't appear in the config files.

To do this you will need to create a "Control Plane" VIP to update the data. This VIP would be internal-facing, source IP restricted and maybe client cert protected (you wouldn't want just anybody accessing it). It wouldn't have a pool associated - just an iRule

when HTTP_REQUEST {
    set key [URI::query [HTTP::uri] "key"]
    set value [URI::query [HTTP::uri] "value"] 

     Set the key value in session table
    table set $key $value indefinite

    log local0. "Key $key updated by [IP::remote_addr]"
}

Then in other iRules that need to use that data, you would reference [table lookup "blah"], where 'blah' was the value of the 'key' query parameter in the control plane iRule.