Hi,
May be your not getting my point for about Slice tool, Slice tool most proper way to create assets with Proper dimensions. If i will download icons from any thirdy party webistes like Flaticon or iconfinder will get proper dimensions like width & Height should be same if icon size different also. In Adobe Xd after export assets developers can't get proper icons. If icon created 1x size with Height 25 and width 15, it will convert into 2x 50 height & width 30 and 3x size is 75 height & 45 width right ? but when will use with Slice tool 1x size is 25 x 25, 2x size is 50x50 and 3x size is 75x75, i hope this is Proper way to create assets for developers according to my experience, if anything went wrong from my end let me know. please attach image again below for your reference.
I see what you mean. The suggestion is to have a new slice tool in XD like we have in Ps or Ai that allows to create images with custom sizes. That is very helpful future that use all web designer.