There have been hundreds of bugs fixed since then as well as numerous enhancements:
- Added Hacx IWAD support.
- Added text-format Strife conversation system with USDF and ZSDF.
- A textured automap is now available; automap graphics are now externalized.
- Rewritten menu system, which can now be defined with the MENUDEF lump.
- Externalized intermissions to MAPINFO.
- Externalized IWAD detection and gave each IWAD its own independent config section.
- Added FluidSynth and GUS emulator softsynths; fixed issues with TiMidity++.
- Added VOC support and enhanced MIDI playback with support for alternative formats such as XMI and HMI.
- Action specials: Added Ceiling_LowerAndCrushDist and Thing_SetConversation. Added console command to execute specials.
- ACS:
- Increased maximum script number from 999 to 32767.
- Named scripts allow to avoid script number conflicts altogether.
- Added StrCopy, CheckSight, SetActivator, SetPointer functions.
- Added APROP_ScaleX/Y and APROP_Mass actor properties.
- Added temporary dynamic strings variables with StrParam.
- Scripts can now have up to four parameters.
- DECORATE:
- Added A_CheckFlag, A_FaceMaster, A_FaceTracer, A_RadiusGive, A_SetMass, A_SetScale, A_Warp and A_WolfAttack...
- Enhanced A_AlertMonsters, A_BFGSpray, A_Blast, A_CustomBulletAttack, A_FireBullets, A_GunFlash, A_PainAttack, A_RadiusThrust...
- Added an actor pointer manipulation system with several new codepointers...
- Added DOHARMSPECIES, DONTCORPSE, SEEINVISIBLE and INVENTORY.UNTOSSABLE
- Increased damage type flexibility in several ways with DeathType and PainType.
- Added customizable fléchettes.
- Made accuracy and stamina accessible through DECORATE expressions.
- Added ProjectileKickback, Inventory.RestrictedTo and Inventory.ForbiddenTo class-based properties.
- Backported VisibleToTeam, VisibleToPlayerClass and weapon DropItem feature from Skulltag, and enhanced them.
- Added "Fast" keyword for states and deprecated the unreliable FASTER and FASTMELEE flags.
- Added a secret hint system.
- 3D floors support.
- FraggleScript support.
- Voxel support.
- SBARINFO: Ammo 1/2 capacities added to DrawNumber, support for color names in colored text.
- SNDINFO: Added musicalias and attenuation.
- Compatibility: fixed issues with Eternal Doom, Happy Time Circus, Phobos, vanilla compat, Boom compat, light levels higher than 255, Heretic, Strife, MUSINFO, misnamed UDMF field.
- Fixed loading of BMF palettes.
- restart console command.
- Added PALVERS lump for specifying alternate graphics to use with a paletted render, in case the graphics you really want to use look bad without true color.
- Custom IWADs no longer need to include a COLORMAP, as ZDoom will generate one automatically if it is missing.
- Fixed vertical tiling of non-power-of-two textures.

I have been using GZDoom purely for 3D floors, now my mods will mostly only need "regular" ZDoom. Of course I'll have to revisit any areas using sloped 3D floors, though I can't recall many.