This article lists the important changes in 1.9.8.
Hexen: Environment Wind sounds (e.g. Map01). There was a sound prioritization issue at map start.
Priorities defined in Sound definitions are used to determine when a sound can stop a previously playing sound from the same emitter (e.g., Cyberdemon and Mastermind alert sound is not cut off by their footsteps).
Heretic: Flying creatures would sometimes get stuck. 3495787
Hexen: disallow warping while dead to prevent zombification.
Graphics and resources
Detect support for non-power-of-two OpenGL textures and don't try to use them if support is not available. Fixes the problem of the game UI/infine background textures being fully white, while the console is drawn ok.
Detail texture contrast normalization is now taken into account when rendering. 3496778
Hexen: The demo IWAD was not being recognized. The buggy version of the demo IWAD is now supported as a separate game mode.