> But \prop_get_gdel:NNN doesn't work, yet. The value of the deleted
 > key is not assigned to the tlp.

hmm, something got messed up when the prop implementation got changed from
using sequences (with tlps) to toks registers. try this one



\def_long:NNn \prop_get_del_aux:w 6
  \tlp_set:Nn #1{#5}
  \quark_if_no_value:NF #1
     \def:Npn \tmp:w ##1#3\q_no_value {#2{#4##1}}
     \tmp:w #6

i haven't updated it yet as I wonder why that change was done. using
\quark_if_no_value:NF is much much faster than \quark_if_no_value:nF as the
later literately scans for \q_no_value which the former just uses the quark
feature that

\tlp_set:Nn \FOO{\BAR}
\if_meaning:NN \FOO \BAR  ->  true if and only if \BAR was a quark