Maybe a picture would help understand what you are seeing, however if it is the same I see on one of my laptops, then it's a physical damage on the LCD screen, maybe it was hit/pressed too hard and it got damaged. No way to fix it AFAIK, only replace it with a good one.

Regards.

It was rather difficult to take a picture that would show this, but I managed to take one that shows something.

Been trying to find any info online.
https://www.youtube.com/watch?v=wrNqM2GO7IU
This is only thing related to something similar online, but the video is bit weird and I don't know if things work the same with a 20 year old lcd.

This exact problem was very common on Dell laptops at least back in the day. Around the Latitude D600 timeframe. Even replacement screens had the same issue regularly.

I did onsite warranty for Dell for a couple years during that time.

From what I could tell, it seems like the layers were not bonded together properly or something caused them to start separating.

I thought it meant the screen was under pressure for some time on this spots. Not sure if anything can be done about it.

Yes, these are pressure spots.

Very common even on some newer laptops, e.g. ThinkPad X220 and X230 are known to have this issue.

Yep, X220 and X230 early screen batches had an issue where they would develop these spots after a relatively short time.

But every LCD screen will tend to develop them with enough age and wear.

I've heard evidence of people who succeeded in removing them, by taking the panel apart and sanding it a bit from the inside, but I would never want to try something like this.

Seen similar spots caused by foreign particles that's found their way into the panel, pushing on and deforming the backlighting reflective layer. Usually, it's not all that hard to disassemble or put together old panels as long as you're careful, could be worth the trouble to open it and have a look.

Be aware that the CCFL backlighting driver works with high voltages, and that the caps might hold charge for considerable time after powering off.

The laptop is Thinkpad T21. I saw some panels sold for it on ebay but they didn't seem like they were in very good condition.

I have completely disassemble at least two displays in the past with those spots and it was some kind of corrosion on the layer inside display.