Every now and then this topic comes up, why shouldn't you check in commented out code?
It's so nice an convenient, then we have the code right there if we need it.
Well, yes, that may seem convenient to you at the time of writing, but this has several disadvantages:
The VCS remembers it for you
If the code you commented out was some previous implementation, this information is safely kept in the version control system. It's easy to resurrect old code by using the VCS. That's what it's there for!
Less is more
The easiest code to read and understand is no code. If some code is commented out, the reader will wonder if this was intentionally commented out. The reader will read the commented out code and will try to set it into the context in which it is written, compare it to the running code and so on.
The less code there is to read and understand, the easier it is to maintain and extend the code at hand.
Commented code is hard to delete
Many people have a hard time deleting code. It feels dangerous. If it's commented out, they may think it's better to leave it in "just in case". We've got commented out code in our source tree being more than 10 years old. In my experience, commented out code never gets commented in again.
Housekeeping
Checking in commented out code also creates larger than necessary commit diffs. People reading the diffs are put through unnecessary work as they have to ensure that the commented out block is of no interest and can safely be ignored.
Conclusion
Before committing your code changes to the code repository, have a look at the diff to be committed and verify that it doesn't contain any commented out code blocks.
It'll save everyone working on the code base a lot of time and effort. Trust me 😃
