It is indeed the case that a number of OBSs have retained RMT membership, where they were formerly conductors. The RMT takes subs from them as "ex-conductors", although, if I remember correctly, there is unsurprisingly still no agreement between GTR and the RMT about specific unions' reps representing anyone on an OBS contract. Staff in RPI/RPO grades were also sometimes members of the RMT and some of these still appear to be subscribed in some way.
The RMT have failed to recognise the OBS grade in and of itself, and this makes something of a tricky situation if new-entrant OBSs want to be RMT members. I don't believe there's much interest by the RMT or the newer OBSs in becoming union members. Those staff who have entered the OBS role without previously being in the revenue or conductor roles would have been unaffected by the original dispute, but would also have been well aware of the problems at GTR before they joined the company, given the news coverage. Thus the RMT will be well aware that these individuals have joined the OBS grade of their own free will and will have accepted the T&Cs likewise.
A fair proportion of the conductors retained in their original grade by GTR are not RMT members, either never having been so, or else having dropped out of union membership during the recent dispute. I believe a slim majority are still in the union.
TSSA also have a handful of ex-conductor OBS members, but either no conductors or very few - perhaps a couple at Eastbourne.