I think it's longstanding - but the examples we've had are almost all of buses serving different NR (LUL for Hammersmith-Heathrow) stations and I doubt they abstract any significant local traffic.Is it because of the general bus deregulation that this all is possible in the UK? Over here in my area in Germany there's currently a longer bus substitution and that goes past all the major innercity interchanges and just stops close to railway stations (which are not in really useful places due to topography). Otherwise it would be considered an unlawful competition to the city buses. There are sometimes to this rule exceptions in rural areas where stations might really be way off from where people live/work/shop, but in urban areas it's quite strict.
One we've missed (I think) is Newbury Park LUL which is a regular terminus for Colchester line RRBs when Liverpool St is closed.