Honestly the answer depends on the type of glass. Highly crystalline glass likely won’t ever warp in our lifetime if ever.
Likely the process of making glass in the past was why they weren’t perfectly level. Some spots would be a little thicker than the rest. If you think about it the bubbles would shift also if the glass was properly amorphous. It does however display very slight changes in structure likely having to do with light instead of a difference in solid/liquid state. Older glass could have shifted with time under the right circumstances though and that depends on how long the glass set before being installed, the level of heat exposure, if there’s hotspots from the sun as well, and how completely they melted the glass before setting. If they added anything to the glass at a later point too. Corning glass actually talks all about this on their youtube channel. Check it out if you want to learn about glass blowing and they have glassmaking facts thrown in.