If you reinstall Skyrim and remove all your mods, but keep your saves and then load your saved game that previously had mods, should there be any mod effects left in the game?

I did this and some guards were still altered by a mod I removed and the grass textures still looked altered from Vurt's Flora Overhaul. Neither mods were active.

Most choose mods when the start a game, and not switch up mods on a character. Things can get really wonky and not even show up for some time.

Some stuff is carried over onto your save file, which can cause problems, but most don't.

Yes, some mods' changes are written to a save file, so some effects may be possible. I once in Fallout 3 got a bug from an overhaul i wasn't using after a clean install because i used a start of the game save where i had had it installed

Grass definetly shouldn't be one of them, though.

So deleting the save file should essentially set it as completely new, right?

I just wanted to start new, update all my mods and go on.