So I've got this really stupid/clever idea...
Just follow me down this foolish rabbit hole....

With the old first gen Altima automatic transmission you get this irritating Overdrive on/off function.
Why Nissan thought it would be a good idea to lock you into only three gears in OD Off is beyond me.
What makes infinitely more sense is to give you all four gears and have that "mysterious" torque converter lockup "fifth gear" be optional.

It's really irritating to be driving in the city at 35mph and that darn torque converter tries to lockup on you which drops your rpm to a bare few hundred over idle.... then it can't decide whether to stay locked up or drop back to third... oh, oh maybe fourth! Annoying.

I propose a torque converter lockup solenoid bypass switch. You could essentially have a highway and city mode. Bypass the lockup for city driving, you'd still be able to access the fourth gear. Flip this theoretical switch to highway mode, and you re-establish the function of the lockup solenoid. (you could apply this to many different cars/manufacturers)

I'm curious what your thoughts are....

I have done this on an Olds Cierra FWD, just inserted a switch to "break" the ECM signal that energizes the O/D relay for magnetic lockup. BUT ! IF I didn't do EXTENSIVE research , circuit AND transmission study of fluid ckts and following the pressure lines/actuations , especially reading up on IF SOMEONE DONE IT BEFORE SUCCESSFULLY !!!
More than one model of tranny will NOT SUPPLY PROPER LUBRICATION if inturrupted - as a "Mod Feature"
In this way .

Intuitively you would think that simply cutting the signal to the lockup is all that is happening as a result of your mod-ding a sophisticated system.What's REALLY going on under the radar , something that as the OP expresses as " you would- or wouldn't EXPECT " X from those engineers. Can terminate "with extreme prejudice"
your trans.

There is some confusion here about the difference in converter lockup and having OD. The OD drop is quite a bit more than converter lockup, which is commonly only a couple hundred rpm.

Like here.....

'Bypass the lockup for city driving, you'd still be able to access the fourth gear. Flip this theoretical switch to highway mode, and you re-establish the function of the lockup solenoid.'

There is very little difference there. Yet OP thinks it's doing something big.

And here...........

'...then it can't decide whether to stay locked up or drop back to third...'

Again confused about what is happening there, the two are not opposites, rather, close to parallel actions. And missing the middle action, or, stay locked up (in 4th), (or drop back into 4th not locked up), or drop back into third instead. And the 3rd may even have 2 positions too, locked and unlocked depending on throttle %. Most ATX have lockup in both 3 AND 4.

Yep, this is always a huge can of worms!
I've run across many conversations about bypassing the lockup solenoid for many different cars. Doesn't seem to be anyone who has tried this with an old 97 Altima that i can find online.... or at least decided to tell the world about it.

Thanks for taking your time to chime in. It would be an interesting experiment.
I obviously didn't explain it clearly enough. Tough to do if you can't sit in the drivers seat and experience it yourself.

I'll probably leave this on the shelf of silly ideas anyway. But I should add the caveat of, don't even think about this if you don't understand how your specific transmission, for your car, and model year, functions. Or if you don't have any basic AT physics understanding, or just simple fluid dynamics in a closed system.

So I did a bit of road testing and paying closer attention to the shifts and when the TC locks up in fourth gear.
So when the TC locks up in fourth during normal slow city driving at the 35-40mph range it drops from 2000rpm to about 1500rpm. Hardly an insignificant drop.
Where the annoying part comes into play - that rpm (1500) is pretty much where the computer rests the idle for a rolling idle, at speed idle with closed throttle, whatever term you prefer to use. To maintain this speed on a flat road is where this old car just gets funky and it starts to have trouble deciding whether to disengage the TC and stay in fourth or drop back down to third. Essentially it just feels unstable and jittery. Hard to describe it better than that. Maybe absurdly delicate gas pedal play is a good way to describe it....
One thing I did notice the other day is that my throttle cable is pretty loose where it connects to the throttle body and butterfly. Not alarmingly so, but a little....I could probably tighten that up very slightly just to keep the TPS sending out a signal to not keep trying to go to the highest "gear". Could very well be that a minor adjustment there could solve the problem. Could be a dirty old TPS sending funky info back to the ECU...
Old cars are so much fun!