Regarding that "spare line", the IBM 3276 documentation gives the following screen dimensions:

  Models 1 and 11   960 - character display  12 rows of 80 characters
  Models 2 and 12  1920 - character display  24 rows of 80 characters
  Models 3 and 13  2560 - character display  32 rows of 80 characters
  Models 4 and 14  3440 - character display  43 rows of 80 characters

However, these are the addressable screen locations, not including the "Operator Information Area" at the bottom of the screen, which we would nowadays recognize as a status line. (E.g., clearing the screen does not include the Operator Information Area, while clearing all available display characters.)

So, good point. Without this extra line, it would have probably been difficult to integrate a PC into an existing 3270 installation and its programming.

ADDS did call it a "Status Line" in their documentation. It was purely terminal status, as there was no way to control its contents with escape codes.

But if IBM were also using a 25th line for it's "Operator Information Area" on some of their terminals, then it's a good bet that where the IBM PC got it's 25 line standard from.

As I understand it (from documentation only) the Operator Information Area is exclusively system controlled and features its own characters/glyphs, which are not available for general use. Like a status line, it reflects the state of the system, keyboard modes, and so on, and is distinct from the general use area. So it's pretty much the same as a hardware controlled status line, but for a complex system.

BTW, another, entirely different point may have been in what we may call "compatible appearances" (somewhat related to UX): meaning, even if it was about entirely different software, written especially for the PC, IBM may have still preferred to provide this in a format already established by their existing terminals and the software running on those.

I did work experience at an IBM "shop" in the 80s, at a time when the PC and PC XT had just come out. They ran IBM minicomputers and used both 3270 terminals, and PCs fitted with an expansion card with a coax connection, to interface with the local mainframe, and the head office one (via some kind of expensive leased line).

The 3270 had all the function keys "soft functions" on the 25th line. The terminal emulator on the PC had the same, and it was basically necessary for compatibility with the terminal applications they used. Being IBM, I have no doubt it was heavily standardised. Probably the precursor to Common User Access (CUA)?

A bit odd that later EGA and VGA didn't have the 32 row mode, and had 43, but no 44-line mode.

I'm assuming that, by then, it was clear that "metal" 327x's were a dying breed that couldn't be sold at a profit for more than a PC.

Are you saying IBM did Spinal Tap?

“Well, it’s one taller, isn’t it? It’s not 24. You see, most blokes, you know, will be typing at 24...Where can you go from there? Where?” Marty asks, “Why don’t you make 24 a little wider? Make that the top number and make that a little wider?” Confused, Nigel just says, “These go to 25.”