I think we should include this but with a reasonable deprecation warning.

Somewhat related: It would be great to have provenance information on the factors in in the units file. A more extensible file format would help realizing this (#1561).

For the current format I have a few ideas that are easy to implement:

Inline

plain

bigstick = 4 stick # The one and only manual

with colon (similar to how attributes were documented at some point)

bigstick = 4 stick #: The one and only manual

with colon and a keyword (like # type :)

bigstick = 4 stick # source: The one and only manual

blocks

Current block syntax

@source The one and only manual
    # everything within this block has the same source
    bigstick = 4 stick
@end

Adding and removing a default source

# source: The one and only manual
# everything without source within this block has the same source

bigstick = 4 stick

# source: 
# This line above set it to blank

In addition, all these could be connected to some form of indirection that could be useful if there is a lot of repetition.

bigstick = 4 stick # source: [1]

# source [1]: The one and only manual

I do not like@source, while consistent with everything else in the definitions file it would make it uglier and difficult to read. I favour a combination of plain and block.

For the current format I like the last form most. It is easy to remember and familiar to most as it matches citing in literature.

I put up a PR for a first pass at this update. Let me know what you'd like to see from there.