Whether the world has moved on or not is irrelevant. The OP has an OBD1 system, period, so working within that is fully capable of doing so and getting the OP's desired outcome, with some effort.
Again there's no mail order chip that will do what the OP wants, or at least not very effectively. Yes, leaning out a VE table (somewhat) arbitrarily can increase mileage, because less fuel is being delivered, it may also cause other issues, like rough running, over heating, difficulty driving etc. So there's nothing the OP can provide (to me) to provide a chip to him to do what they want (besides their truck), again, again, again, this will be an iterative process that needs to be done over a long period of time, which is best done by the OP or working with someone locally that has the ability. Again, I don't provide mail order tunes, because there's just too much missed when you can't see, feel, hear what the engine is doing, especially when it's a drivability parameter, and not just a power parameter that is being adjusted. If I was to tune the OP's truck, I would need the truck, for an extended period of time, to test before, during and after adjustments to verify improvements, which again is an iterative process, that doesn't happen quickly. When I tune a vehicle I do try to tune for efficiency even when I have a short period of time with the vehicle, but gaining every last MPG has never been the focus, typically it's been drivability, meaning good manners, good idle and power. MPG as the focus is not typically something that anyone has asked me to tune for, but I try to get the tunes as efficient as possible anyway, so mileage just comes as a natural part of that.
But I disagree that this discussion doesn't help the OP, since I'm pointing out that what they want to do ISN'T a waste of time or money and is completely possible to get their desired outcome, for a pretty modest investment, it will just take some effort on the OP's part is all.
If the OP was to follow other's advice in this thread they would sell their truck and buy something new, because any work performed on the OP's current truck is a waste of time, whereas I know the opposite to be true, but it just isn't as easy as dropping in a magical increase mileage chip.
As someone who tunes OBD1 ECMs I feel that I need to correct incorrect information about it when someone posts it. I have lots of real hands on experience with OBD1 (Even helped start one of the go to forums for OBD1 GM tuning years ago, and help moderate another), I have made combinations work that so called experts said would never work, whereas it seems that other people in this thread don't and are just repeating bad information that has been said by other people with little to no experience with OBD1, that doesn't help anyone looking to tune their OBD1 vehicle. There's no need to throw thousands of dollars and unnecessary hardware at a simple change. I have seen this far too often, I also tune plenty of aftermarket systems and see people spending WAY too much on systems that they will never make full or even use half of the capabilities of that system, because somebody on the interweebs told them they needed to buy the wizbang device to solve all of their issues.
In the end the OP will do what they feel best for themselves, I just can't stand by when bad information is posted misleading people.