The obvious answer is CLI stuff like bash, grep, sed, xargs, tar, etc. The shell suite found on Linux and macOS systems can contain code dating back to the 1980s. There’s some really ancient code in there. No need to change it.
For UI apps there’s not much I use that’s that old.
Trunk Notes on iOS. It even runs on mac since Apple Silicon.
Some may be older. Ones I actually port to new machines and use are the perl "ren" script and the rdb relational database (perl) by Hobbs.
Source:
https://github.com/ironsmith58/RDB/tree/master
Article:
https://seann.herdejurgen.com/samag.com/html/v11/i01/a6.htm
Docs as pdf:
https://casegroup.rutgers.edu/lnotes/ccb550/spring20/RDB.pdf
Assuming you mean phone, I think every app I use is more than 10 years old. The app ecosystem has not been worth exploring in many years for me.
I bet most of the app installs these days are just kids tapping misleading ads they see in bad games.
cmd.exe came out in '82, so it is 7 years ahead of bash which was '89, so I think it wins.
Total Commander - https://www.ghisler.com/
Swiss army knife for file/SFTP/… operations on Windows
On windows it is still my go-to filemanager (although double commander does the same)
Gmail is pretty ancient now. Webstorm too.
vi(m), sc (the spreatsheet), awk, Perl (for quick oneliners), mutt, grep, tcpdump
Excel and Word from Office 97.
For me it is an old version of FlyingLogic before they went subscription only.
WinSCP and IrfanView.
The obvious answer is CLI stuff like bash, grep, sed, xargs, tar, etc. The shell suite found on Linux and macOS systems can contain code dating back to the 1980s. There’s some really ancient code in there. No need to change it.
For UI apps there’s not much I use that’s that old.
my mail user agent says: Mutt/1.5.21 (2010-09-15)