Powder coating can/will damage a liner. If not the abrasive entering the tank, then the heating/curing process of an installed liner will.
Yes, I believe every BMW tank since the early 60's has a factory installed "treatment" on the interior. Its not so much a liner as is considered today, but it was used to protect the metal from its production date.
Many modern bikes don't use liners as they use a fuel bladder beneath a tank shroud. And liners aren't a "band-aid" fix. They're a practical method of solving thinned metal issues due to rust, pinholes, or as a prevention of rust. If you have a stock paint job but some pinholes, then welding or brazing on a stock original paint job is not a good idea. Unless of course you don't care about the value of original paint...