I appreciate your pedantry, and I wonder if you know of a widely used computer system which genuinely implements the GMT time zone as something different from U

I appreciate your pedantry, and I wonder if you know of a widely used computer system which genuinely implements the GMT time zone as something different from UTC+0. There may be some niche astronomical or legal(?) applications which have to keep track of the potential 0.9 seconds skew, but my impression is that any app which claims to support a "GMT" (or Europe/London) option is actually only supporting "GMT-ish".