LEXXIUS

If it gets hot in your pocket and drains when idling, it's possible that a cryptominer somehow got on the phone. Unfortunately, as far as I've read, it's not easy to find it. Or maybe the battery went bad prematurely... One thing to make completely sure if this is a software or a hardware issue: Factory Reset, set it up again and test it.
 
I also love my 256Gb S9+ but the battery doesn't idle well, on my old Axon 7 (1 year 9 months) with a very used up battery, it only drained at about 1-2% in deep sleep, the S9+ loses much more, my battery graph decline is much steeper (but it's a consistent decline, resembling a straight line) compared to the old Axon 7 (much flatter in deep sleep but drops when used pretty fast).
 
Also at the end of my workday (not what I expected from a bigger brand new battery):
old Axon 7: 50 - 55% left
S9+: 50 - 65% left

That's why I don't like smartwatches. Also their battery life and limited usefulness...

If it gets hot in your pocket and drains when idling, it's possible that a cryptominer somehow got on the phone. Unfortunately, as far as I've read, it's not easy to find it. Or maybe the battery went bad prematurely... One thing to make completely sure if this is a software or a hardware issue: Factory Reset, set it up again and test it.
 
I also love my 256Gb S9+ but the battery doesn't idle well, on my old Axon 7 (1 year 9 months) with a very used up battery, it only drained at about 1-2% in deep sleep, the S9+ loses much more, my battery graph decline is much steeper (but it's a consistent decline, resembling a straight line) compared to the old Axon 7 (much flatter in deep sleep but drops when used pretty fast).
 
Also at the end of my workday (not what I expected from a bigger brand new battery):
old Axon 7: 50 - 55% left
S9+: 50 - 65% left

I pictured Linus carrying multiple high-end phones around but when making a call he takes out a flip phone (or an 80's brick, which might be even funnier)!

That's why I don't like smartwatches. Also their battery life and limited usefulness...

When the S8 was officially announced, Samsung said, the battery will retain 95% of its capacity after 2 years. Looking at actual user experiences (reddit) it seems mixed, like 60% (my rough estimation) reported their battery is still as good as when they got the phone. I believe this is achieved by only charging the battery only to 80% (sweetspot for usabe charge and degradation), there are research findings that say using the battery in the range of 20% to 80% prolongs its life drastically.
This could also be a hint for that statement being true: When rooting any of the latest S8 & S9 phones and using custom roms, a lot of people report their batteries only charging to 80%. This could mean the software displays the full capacity but the hardware still limits it.

Did you try doing a hard reset on Linus' phone and see if it still behaved the same way?
Maybe he just has a hidden cryptominer on his phone or something...