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 4.57 ( - CURRENT VERSION - )

(Release date: 2021-01-13)

• Improved option "Try registry changes" (Properties->Extensions)
• Option to use new Windows File-Open dialog (Properties->Misc)
• Option to create only Shortcuts (LNK file) in File-Copy dialog
• New Camera RAW PlugIn (based on LibRAW, thanks to Alex Tutubalin)
• Support for WPG format (PlugIn, WPG version 1; thanks to Flurmy)
• Improved GIF reading
• PCX loading bug fixed (thanks to @DmitriyMelikov, CVE-2020-35133)
• MNG PlugIn loading bug fixed (thanks to ThanhTP)
• PCX format moved to FORMATS PlugIn
• Several PlugIns are changed/updated, please install the newest versions:
  https://www.irfanview.com/plugins.htm

[ Older versions - History]