Swiss Stadler FLIRTs do that, never seen it in the UK, I guess the concern is that people will press it, forget about it, lean on the door then when it opens fall out.
It has a secondary purpose in that pressing it requests a stop at a request stop.
Will be interesting to see if the UK FLIRTs have that feature, I expect not. I don't think the DB AG ones do.
That's actually a general thing on both Swiss trains and buses. Certainly the modern Stalder and Siemens train do (haven't tested the eternally delayed Bombardier's), in addition to the IC2000 double deckers and ICN, and also older single-deck S-Bahn stock. The only real exception is the single-deck EW IV carriage that have levers on the inside, those don't activate until doors are unlocked. (If the buttons aren't illuminated, they'll illuminate once you press them.)
Many of the buses where I live even have lights above the door to indicate they will open (only the most local buttons activate the door, other stop buttons in the bus activate the main stop light). They were installed on all the older buses, disappeared for a while as low-floor buses were introduced, and have reappeared recently on new deliveries.
But yes, you guessed it, my issue is that people don't realise that pressing the button once is enough. As well as people who block the button without realising that it needs to have been pressed at least once or the door won't open either (that happens once a month).