[njt]

Both Subversion[1] and CVS[2] had keyword substitution, which replaced those tags with useful information like the commit id, author, date, etc.

They were very useful when you were looking at a source file, to see what version of that file you had.

Git had something similar with Git Attributes[3], but AFAIK, they were just references to blob ids, so they never really took off.

For git, I now use tags (and versioning based on tags), that more or less replaced svn/cvs keyword substitution in the git ecosystem.

[1] - https://svnbook.red-bean.com/en/1.7/svn.advanced.props.speci...

[2] - https://www.gnu.org/software/trans-coord/manual/cvs/html_nod...

[3] - https://git-scm.com/book/en/v2/Customizing-Git-Git-Attribute...