Being the work for compatibility something really important, and as it requires a lot of work, we could think about creating specific Milestones for it. We would usually use it for versions, but it would make sense in this case.
Unfortunately the Trac ticketing system doesn't support dependencies. This is something I've always missed on it. It seams they are working on it.
The "Keywords" have other usage, and the most important thing about dependencies is the possibility of quickly understand if the dependencies are opened or closed.
This behavior can be accomplished in the bodies of tickets descriptions or comments, by simply creating tickets links. So, as we all are able to make changes to tickets descriptions, we could add an horizontal line at the very end of this description and add two lines, one for "Depends on" and other for "Blocks", like the sample I'm adding in this ticket. What do you think?
