First time this had happened: there was an RPI doing ticket checks (Oysters included) between St Pancras and Farringdon this morning on Thameslink. For all Oyster passengers, he was asking them where they were going and reminding them that East Croydon would be the limit.
I've had it a few times, but not with the helpful advice you mention. I would be surprised if Oyster wasn't checked, to be honest, as many people would be using it. Likewise Contactless etc.
In the 6 years I've been travelling to London Bridge I've never been checked on board.
To be fair, I should imagine it is a lower priority from Sydenham up to London Bridge/New Cross Gate due to ticket barriers now being in regular full-time use at the majority of popular destinations. I can appreciate there will be barrier jumpers, people with incorrect travelcard zones, etc., but I hope you can see why this route would be of a lower priority than, say, some of the smaller stations between West Croydon and Dorking, South Bermondsey - Tulse Hill, or further down the line in the Anerley area.
That said, the other examples I've given aren't exactly teeming with RPIs either, bar Peckham Rye.
I wish Southern would be more pro-active in checking 1st class tickets.
It's something conductors and RPIs are able to use their judgement to prioritise above other checks if that's the only thing possible. Sadly the trains are often too crowded to move people out, or get revenue staff in, anyway - I have been towards the less crowded parts (!) of a 12 car set where it wouldn't really be possible for the guard to leave the cab to go and check, because there are three people in the gangway between them and First Class and goodness only knows how many in the aisles beyond that.
Wherever services can be lengthened in future, I'm sure checking will gradually become more possible. With standing/circulation room improved in the Class 700s, for example. But with a 377 where many of the Standard seats are in 3+2 layout with loads of people between them, you try squeezing down to check First, and then go 5 coaches back for your dispatch position... sometimes it can be done, but often sadly not.