Undo and recycle bin might sound similar when you look at it from the user's point of view since they both can be used to undo a delete operation for instance. Though, technically they differ significantly.
The undo functionality has been in core since forever and all we need is a Plone REST API endpoint to expose this and a Plone 6 (Volto) control panel to show the undo operations (and maybe allow to filter and search on the db operations).
Recycle bin never has been in core and there are numerous implementations by senior members of the Plone community and we were never able to integrate that into core. Recycle bin is a very complex topic IMHO because there are multiple ways to implement it and the community never reached a conclusion what's the "right way" of doing it.
If I were a GSoC student (and I actually was in 2009 and I was a mentor 5-6 times since then) I would rather go with the undo control panel. It is a lot easier to implement and it is clear what needs to be done.
Recycle bin is something where the mentor and the student have to put considerable effort into drafting the solution. If you are up for that, I'd suggest talking to @tkimnguyen, since he proposed that project.
Personally, I am very much interested in having a recycle bin implementation. Though, I don't really see how a GSoC student could accomplish something that experienced Plone core developers (some of the smartest people I know) did not accomplish (at least not bringing it into the core). That's just my personal opinion though. Other community members might have a clearer view of what needs to be done here.