Well there are actually several answers to these questions.
As the new versions of these standards are released the languages generally become more powerful and more secure. At least that's what happens when the people who work on the standard know what they are doing. Of course no implementation is perfect and it may be discovered that a particular version of the compiler or web browser either doesn't implement the standard quite right or it has a security hole in its implementation that needs to be patched. So the environments in which the standards are applied are updated more frequently than the standards themselves are. With most of these standards only the last one or two major versions of the standard are actually supported and environments that support only older versions of the standards cease to be patched even if a major security hole is discovered. It is simply expected that those writing code will keep track of where the standards identify something as deprecated and will arrange to replace that code with the new equivalent as soon as possible so that when support for that command is completely removed in the following version of the standard that their code will no longer use the command that at that point no longer exists.
The time saved in maintaining code that follows the latest standard will soon exceed the extra time needed to learn how to write to the latest standard properly and to create new code that way instead of jumbling things together the way that the earlier standard required. With languages that run in environments other than a web browser the people writing the standards and creating the environments to implement the standards realise the importance of keeping up to date with the latest standard. Only a few novices running these environments on their local computer even have the opportunity to continue using antiquated versions. Only with the web where the novices make up the majority of the people writing code has the situation arisen where new code is being written using long obsolete versions of the standards. Only with web browsers has it ben necessary to modify the standards to require that commands that should have been deleted long ago still need to be supported because there are simply too many places where those commands are still used.
This article written by Stephen Chapman, Felgall Pty Ltd.