You might be surprised. First, I run EndeavourOS daily on a 2008 iMac and it not only runs but is very useful. I browse the web, watch YouTube, video conference, create office docs, play older games, do basic programming, run Docker ( well, Podman ), watch movies, read ebooks, edit audio, etc. With EOS, all my software versions are up to the minute.
The reason I use that machine so much is because of where it is. I like that spot. The reason I have not put something else there is precisely because it works so well that I have no reason to. I use XFCE to keep it light and have to restart the web browser from time-to-time to free up RAM but it is fine.
The first 64 bit Intel chips were in 2007 but AMD released the K8 way back in 2003. I do not have one to try but my guess is I could install the most recent EndeavourOS on such a machine.
That gets us to 21 years ago pretty easily.
You would be amazed at the upgradability of older hardware. You can drop 16 GB of RAM and an SSD in a 2009 MacBook.
However, you can run a 100% modern Linux distro on hardware much older than that. Many distros, including Debian, have 32 bit versions that support Pentium Pro and up. Most software available in regular Debian is also available in the 32 bit versions. The package release numbers are the same. So, totally up to date and modern software. You can run Debian 12 on 32 bit processors.
That takes us all the way back to hardware from 1995! That is just 14 years after the first IBM PC!
In practice, the biggest problem is going to be RAM. Anything below 6 GB for 64 bit and 4 GB for 32 bit is going to struggle with the size of modern software ( especially web browsers! ).
I am not sure how far back you have to go before the processor is just too slow for everyday stuff. I would guess around 2003 or so, depending on what you are doing.
Windows XP was introduced 20 years after the sale of the first IBM PC in 1981.
It has one been 23 years since then!
Things certainly changed a lot more before than after.