I've seen it announced a couple times on South Eastern on the evening peak Orpington extras which said something like "First Class can be used by all ticket holders" at Grove Park, when it was a 465/9.
As for South Eastern getting rid of 1st class, a good decision in my eyes. Gives a few extra seats for standard class, although I suspect the only time it will get filled up first will be on 375/9s by those "in the know". I suspect other London operators might follow suit if the DfT are not seeing that many First Class annual season tickets or even sales of 1st class tickets. South Eastern join Greater Anglia, GWR* & Chiltern in getting rid of 1st class (* on the Thames Valley stoppers/branches, bar North Downs).