Thanks for your comments, and thanks to Juri for his suggestion.

Updated patch attached, which incorporates your comments and Juri's 
suggestion.

>> +(@code{make-frame-command}).  A prefix argument undeletes the last deleted
>> +frame, a numerical prefix argument between 0 and 15 undeletes the
>> +corresponding deleted frame, where 0 is the most recently deleted frame.
>
> This is unusual meaning of prefix argument.  Why not use zero for the 
> last, 1 for the one before that, etc.?
>

You mean: 0 for the least recently deleted one, and 15 for the most 
recently deleted one?  So to recover the frame you just deleted by 
accident, you'd have to type C-u 15 C-x 5 u?  That seems unnatural to me, 
but perhaps it's just me.

>
> +(add-hook 'delete-frame-functions #'undeleted-frame--save-deleted-frame)
>
> I'd rather we didn't do that by default.  Several reasons:
>
> . the startup code deletes the terminal frame, so the above means we will always load frameset, which is not a small package, at startup, even if the user has no use for this functionality
> . using add-hook in Emacs's own code _by_default_ is not a good style; hooks are for customizing the default behavior
> . saving configurations of 16 deleted frames _by_default_ means we again impose on all users something that only some of them will use
>
> So I'd suggest instead making this an opt-in feature or maybe even minor 
> mode.  Only when turned on should we save away the deleted frames.
>

A minor mode is another option, indeed.  My feeling is that this feature 
is something about everyone would find useful, and that the cost you 
mention is not that high.  And I solved the problem of the deletion of the 
terminal frame.  So I made it an opt-out minor-mode.

>
> (And did you consider wrapping this into some history-like feature, 
> where users could interactively select which past frame to restore?)
>

Yes, I did consider this, but did not really know if it would be worth 
doing that.  Apparently it is ;-)  It's not yet clear to me how one could 
select one of the frames in a meaningful way.  I'll try to do that later.

>> +The 16 most recently deleted frames can however be undeleted with
>> +`undelete-frame', which see.
>
> The "however" part is "out of the blue" here; I'd drop it.
>

I see what you mean, but it's not out of the blue, it's meant to balance 
the "permanently eliminating" in "Delete FRAME, permanently eliminating it 
from use." two lines above.