You can download old IrfanView versions from these sites:
Oldapps.com or Oldversion.com 

There were numerous changes, here are only the more important things.

Version 4.54 ( - CURRENT VERSION - )

(Release date: 2019-12-12)

• Multiple Undo/Redo steps added (max. 20 steps, Properties->Editing)
• Zoom magnifier added to fullscreen mode
• Option to disable Zoom magnifier (Properties->Viewing)
• Profiles option added to Print dialog
• New option in Properties->Fullscreen: Set horizontal and/or vertical centering
• New option in Print dialog: Set horizontal and/or vertical centering
• Option to set selection border thickness (Properties->Editing)
• Option to enable/disable PlugIns (Help->Installed PlugIns)
• Extended sidecar files option: Copy/Move possible (Properties->File Handling)
• New Canvas dialog option/method: Set aspect ratio
• New option in Watch/Hot Folder dialog: Watch subfolders
• Support for CR3 files (preview only, add CR3 to extensions list in: Properties->Extensions->Load custom types)
• New Lossless JPG Dialog option: Add (or Replace) ICC color profile
• New Thumbnails option: Keep focus in folder tree (after folder change/click)
• Option to show Color Corrections dialog in dark mode
• Fix in Metadata PlugIn (crash with bad EXIF tag in CR2 files; KVE-2019-1184)
• BMP loading bug fixed (thanks to Joshua Faust, CVE-2019-16887)
• JP2 PlugIn loading bug fixed (thanks to Linhlhq)
• ANI PlugIn loading bug fixed (thanks to Linhlhq, CVE-2019-17252)
• RAS PlugIn loading bug fixed (thanks to Linhlhq, CVE-2019-17251)
• EXR PlugIn loading bug fixed (thanks to Linhlhq, CVE-2019-17255)
• DPX PlugIn loading bug fixed (thanks to Linhlhq, CVE-2019-17256)
• WSQ PlugIn loading bug fixed (thanks to Linhlhq, CVE-2019-17250)
• JLS PlugIn loading bug fixed (thanks to Linhlhq, CVE-2019-17253)
• Several PlugIns are changed/updated, please install the newest versions:
  https://www.irfanview.com/plugins.htm

[ Older versions - History]