I can't speak for others but tech is only changing on surface. Inside the thing remains same: collect data, manipulate it and display it. This has never changed and till the point of AI taking over, it won't change.
Till we replace the screens, we will continue to have HTML, CSS and one server side language like say javascript, python, etc etc.
The code you have written in 1997 is only needed to be changed if that code has any server side language changes. That's why many pages made in geocities are still accessible and they render just fine.
Let's move on to complexity. Remember VB based desktop applications? Those applications are now moved to C# Dot net core and QT. How much code per year needs to be changed? That depends on the Operating system in target like say Linux, Windows and Mac.
PHP Side not many big changes happened last 10 years. Except Composer and Laravel. Python is still stuck with Django and 2 or 3 odd frameworks. C# is kind of closer environment even after open source. Java is slow dinosaur that will keep old code, slow and for long time. Javascript seems to be taking off quickly due to node and it has changed more than any other language out there. Lot of modules, frameworks and changes.
So if you are working on Javascript based projects expect the changes on recurring basis. Any other language hardly showing any promise and changes in it's core and frameworks.