Mischon123,
Paste is 1/3 the price of a liquid cooler?
First off, we are talking about TIM in between the CPU chip and the heat spreader. You'd need to DELID to see it.
Anyway, nobody knows why Intel is avoiding solder. You can read this but really nobody knows: https://venturebeat.com/2018/10/08/intel-solder-9900k-i9/
I will say Intel Engineers aren't stupid. It's also not that costly in terms of profit margins. Micro-cracks don't seem like much of an issue as they solder Xeons for servers. So the above suggestion it's about conflict-free materials by the process of elimination may be true. So maybe they just used solder now temporarily for the worst offenders temperature wise and will stop using it again soon.