Good news. Sort of.
I figured out which files control those particles, at least the relevant shader.
go to AoE2DE\resources_common\shaders\
ember.fxo
then go to the d3d11 subfolder
ember_ps_main.pso
ember_vs_main.vso
You can delete them; but if you really want them gone, you should replace them with an empty file of the same name, so that they don’t come back the next time you update.
I tried putting the empty files into a mod, but the game doesn’t want to use the modded ones then. So you have to manually delete/replace them.
If anyone happens to have a backup of the game files from the period during the winter holidays, please let me know. I tried to get the old files from steamdb, but even with those shaders, I couldn’t get the snow particles.
EDIT: I found another way to disable them.
Try this mod: https://www.ageofempires.com/mods/details/15618