How is that if I decide not to sync (with LR Mobile/LRCC) a LR Classic collection anymore its pictures are present in LR mobile /LRCC "All photos" folder anyway???
Collections are the counterpart of Albums in LrCC, so if you stop syncing a collection, you only stop syncing with that album. Images can also be in LrCC without being in an album however, that is why they won't be removed. If you want to remove images from LrCC, then remove them from the 'All Synced Photographs' collection in Lightroom Classic. That is the counterpart of 'All Photos' in LrCC.