That is untrue for almost all software written outside of companies who's primary product is tech or software. I work for a publisher and while there's a ton of disposable microsites and project work (that nobody ever cleans up or documents, so you never know if it's okay to remove), there's also ancient monoliths written in forgotten programming languages that are so important for production and have so many hidden dependencies and unknown users ("random FTP server" being my favorite), that you can barely put on new paint.

I'm writing software with the assumption that it'll be used for at least 30 years there, with a lot of guard rails and transparency/observability mechanisms, because I know the next person working there will thank me.