the session is not deleted
hmm, interesting. I can confirm what you observed. However,
the following identical code does work:
<cfset t.x.s=1>
<cfset t.x.t=2>
<cfset t.x.u=3>
<cfset t.x.v=4>
<cfset t.y.s=5>
<cfset t.y.t=6>
<cfset t.y.u=7>
<cfset t.y.v=8>
<p>Before structdelete</p>
<cfdump var="#t#">
<cfset structdelete(t,"x")>
<p>After structdelete(t,"x")</p>
<cfdump var="#t#">
I therefore suspect Coldfusion will not allow you to delete
such system structures just like that. I'm looking into it.