End Of Service Life hardware list

SLDC is 5 years so the 270 is from 2003

3050 7050 is the current model.

systems development life cycle (SDLC)

Windows 10 is the only game in town for newer processors.  10 works just fine on models as old as the GX620 with Pentium D915.   Not supported does not mean not working.  I never recommend going below this model because the older units are AGP netburst and do not support windows 8 or 10 due to lacking EMT64, LAHF, SAHF, CMPXCHG etc.

Version 1507 (Original Release) Threshold 1  2015
Version 1511 (November Update)  Threshold 2
Version 1607 (Anniversary Update) Redstone 1
Version 1703 (Creators Update) Redstone 2
Version 1709 (Fall Creators Update) Redstone 3
Version 1803 (April 2018 Update) Redstone 4
Version 1809 (October 2018 Update) Redstone 6

Oldest models that have Microcode updates for Spectre and Meltdown are the Optiplex 390 790 990. Therefore I consider these still "serviceable" but EOL anyway based on 5 year SDLC.

This is probably what you are looking for but its not official from dell.

https://engineering.purdue.edu/ECN/Support/KB/Docs/DellModelYears

https://www.dell.com/ee/business/p/optiplex-390/pd

http://i.dell.com/sites/doccontent/corporate/corp-comm/en/Documents/earth-weee-optiplex-desktops.pdf

http://www.dell.com/content/topics/global.aspx/services/main/service_eol?c=us

 Optiplex 390 790 990 from 2011 are obviously past that.

Systems with XP came out in 2002 and ended in 2014.

Systems with VISTA came out in 2007

Systems with windows 7 came out in 2009 so we are already close to 10 years for these.

Windows 8.0 came out in 2012

Windows 10 came out in 2015