Not since
@miklcct brought the validity of such tickets to the attention of GTR, thus resulting in a "negative easement":
I have an annual Southern S-Ticket from London Thameslink to Three Bridges. Tonight, out of mild curiosity as I was at Victoria I scanned it to get the train expecting it to decline and it worked and opened the barrier. For the avoidance of doubt and not wanting to fare evade I also touched my...
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I'm not sure what you mean by "at Southern price"?
If you mean with a route of "Southern Only", most of the fares with this route were withdrawn/harmonised, either when the GX franchise was absorbed into the expanded Southern franchise (2008) or at the formation/expansion of the replacement TSGN franchise (2014/15), from which date "Thameslink Only" fares became legally valid on all trains operated by the company.
Several "Thameslink Only" fares were also withdrawn, and fares harmonised (in a manner that was controversial, leading to the temporary "Redhill Hump" which took years to finally resolve by reducing the cost of the "Any Permitted" fares)