Welcome to ATP!
Unfortunately, I have more questions than suggestions!
1
Are most of the leaves developing those yellow and brown spots, or just the few in the foreground of the photo?
If it were only a few leaves, and I had no idea what was causing the problem, I would cut off all the affected leaves and burn them or throw them away ... just in case insects or some plant disease are taking hold.
But it looks like many leaves have at least some kind of mottling.
2.
Just to make sure: those spots do have dead brown crumbling tissue in the centers, right? They are not just yellowed (chlorosis), they are dead in the center (necrotic).
3.
Can you guess at any relevant changes that occurred in t he last few months? Like increased watering, any fertilizer or water or pH change, dry air, a draft, less sunlight ... ?
4.
Over 20 years, have you root-pruned it and re-potted it (not just "potting up", but real root-pruning-repotting with total replacement of the soilless mix? How often or how recently? Most container plants need periodic root pruning and soil replacement; I assume ficus does also.
If your necrotic leaves have been getting gradually worse for months or years, that seems a likely candidate.
Or, perhaps, insufficient root pruning led to being somewhat root-bound for a long time, but the plants managed to stay superficially healthy until some new, minor stress occurred that broke the camel's back.
Perhaps only a draft or dry, warmer air, or a slight change in watering scheduling.
Dubious speculation:
Excess lime can make iron immobile which could cause chlorosis ... but I would expect everything to turn lighter green before any leaves died. Younger leaves would yellow first, and turn yellow between-the-veins first.
No fertilizer at all for a long time might have a similar effect - but would not make just SOME leaf spots die and leave most of the plant dark green.
If the pot just "stopped draining", or was now being over-watered, the soil mix might have salted up or become chronically water-logged. Either of those could make a plant very unhappy but I would expect more uniform leaf damage, not just specific spots.
I'm guessing. I hope someone who knows your variety of ficus chimes in.
You have the biggest, woodiest, tree-iest ficus I ever saw and knew that it was a ficus. I used to think they were just "little potted plants", not trees!
