This is only true if people stick to a version of a language and don't upgrade.

If you upgrade then, for example, you can't run all of your PHP 5 code in PHP 8, most of it you can but you will have to change the parts that are broken, which are the areas that are repaired in PHP 8.

Same goes for other languages like C# or Python

They don't "need" to maintain backwards compatibility. Several major languages have broken it by now.

Programming languages deprecate features in standard libraries all the time. As PHP did, causing many PHP 5.3 applications to fail catastrophically once the warnings added to PHP 5.4, 5.5, and 5.6 were turned into errors. Of course, maintained software rarely ever runs into this issue.

The standard libraries were the lacking part in PHP. The language itself was never a serious problem.

They are software products like anything else.