NWA moved DC10-40 operated NW34/35 from PIK-BOS to GLA-BOS in May-90, was dropped late 1994 as NWA fed their new AMS hub with KLM.
Air Canada moved PIK-YYZ May-90, went seasonal, then was dropped by end 2005(?)
American flew GLA-ORD May-90, later went seasonal and also latterly dropped.
BA only flew to JFK at weekends from Aug-90 with TriStars then 767s, then moved to Newark, still Fri-Sun only, moved back to JFK when BA passed the route to BA Regional, dropped to 757s, then flew GLA-JFK-BOS (flew over BOS to get to JFK!) then went seasonal on the 767 before ending.
United launched GLA-IAD Aug-93 and flew a year before splitting loads with AMS and ending Jan95.
As STAR Alliance was the way forward, United could then feed their new LHR flights with BMI with no need to fly to Scotland where yields were awful. Too many business travelers still used the many more options over LHR.
By 1998 when Continental launched GLA-EWR, competition had died down, and now UNITED have dropped GLA once again, albeit to seasonal.