There were "mind the gap" signs on the walls as well if I recall. They've been around since the early-mid 2010's, ever since 5 car trains saw use which was fairly frequent at the time.
Both Central and Churchill have this problem to a degree. I'm not sure if it was originally a design issue, but, because of the location of turnouts for the crossovers the platforms taper back a bit. I believe with a 5 car U2 this was fine, less so with a SD160 which overall are slightly longer. Over a 5 car train, the length over couplers is 2.7m longer then a U2.
The northbound platform at Central I believe was fine, with the turnout positioned a little bit further south than the southbound track and just had the signage added.
The southbound platform at Central was not ok. As a result, at the north end (east end geographically) ETS moved the platform gate further into the tunnel, and essentially extended the platform into the tunnel, with a big sign warning that that extension is only for disembarking passengers. A U2 more or less still fits within the station, but with an SD160 train you certainly do get to make use of the platform extension.
Churchill was less exciting, but, I believe the platform gates at the north end were pushed back slightly with new steps to track level, to ever so slightly lengthen the platform and then the signage installed at the south end, although I don't believe the signage was installed initially. I should have photos of the changes when they were new.
I wouldn't doubt that "mind the gap" was inspired by the London Underground.