Linux 2.4 was pretty good, but even most of my IoT junk runs 2.6 which isn't quite 20 years old.
MySQL 4.0 was a lot better than 3.23 IIRC, and isn't quite 20 years old either.
IMHO, it's not so bad for the old version to become unrestricted; more recent changes can still be restricted. And 20 years of updates are often compelling enough to submit to restrictions.
What number seems right to you though? 95 years for work for hire or death of author + 70 (which is gonna be fun to track down for open source works with many author) seems rather long to me.
And I would argue that _lots_ of the code in today's shipping products (which I listed, and others) use lots of code laid down more than 20 years ago.
Sure there's lots more in Linux now, but lots of the code is also basically unchanged from 20 years ago. It's not like GREP gets a complete rewrite every 5 years...
I'm not saying that I'd be up for using a Linux install from 20 years ago, but hey "any code" is a lot more than "complete install".
All the following are still actively using code that I'd more than 20 years old;
Gnu user tools, as used in Gnu Linux. Linux kernel. Pretty much all databases. (sqlite for example is 21 years old). Much of Windows, and Ms office.
Not to mention mountains of my own code, still in production. 20 years is nothing in coding terms.
Most developrs I know are small or single, and have one or two products they've been working on for over 20 years.