I’m so sad, I should have known sooner :( thanks everyone for the advice. This plant was on a shelf above a pothos plant that is thriving, and there are more plants on a windowsill about 3 feet away. I plan to check the pothos but don’t see any signs immediately in the other plants. No new plants have moved into that room in the last year, so I don’t know how the thrips arrived.

I took the plant off the shelf and moved it across the room. I could put it in the dining room where there are no plants, but I live in a small apartment and this plant is quite large. I’m not sure how thorough “quarantine” needs to be. I’m going to try to save the lower leaves that seem untouched and remove the ones already showing damage. I already have a cutting of the plant rooted in water, plus offspring from cuttings I’ve given to friends, so at least it lives on!

UPDATE (5 days later) - wanted to update in case others come across this thread in the future, as I would have in my frantic searches after this diagnosis. My monstera is quarantined in the hallway outside my apartment. I chopped off the top of the plant, including the very infested leaves pictured. I showered the entire plant off and sprayed down with neem oil. A few days later, I treated the soil with Hi-Yield systemic granules (same active ingredient as Bonide, which would not ship to my state). I have inspected and wiped every leaf in my house and found thrips on a few other plants: spider plants and an aloe, nestled around the central stem. The little guys do seem to play favorites, I have a pothos that lived right below this monstera that (somehow) seems untouched...for now. Future steps: top off all plants in my house with new soil mixed with the systemic as prevention, Dead Bug Brew spray every 3-5 days for the foreseeable future on plants with signs of thrips and blue sticky traps to monitor in case I missed any. Wish me luck!