Ah yes it looks like they did say that support for 32bit Windows is stopping soon:
- The Verge - Steam is dropping Windows 32-bit support in 2026
- Tom's Hardware - Valve to drop Steam support for 32-bit Windows versions next year
- Valve FAQ - Windows 32-bit OS Support
Thanks I had missed this completely!
I assume this is the first step for them to move the client itself to being a 64bit executable later.