Meanski from Slash here.
This change can be made by modifying the patch_d2.mpq on the D2GS side of things, a lot of changes made within these files sometimes do need to have client side patching (your players will also need the same patch file) but luckily with droprate changes - these can be done all server side without any client side patching.
You'll need two programs - MPQ Editor and table editor, also with the MPQ editor you'll need the 1.13C (assuming that is the version you are running) listfile - this means the MPQ editor can understand each file in the patch MPQ. All of this can be downloaded from Phrozen Keep.
Ladik's MPQ Editor: www.zezula.net/en/mpq/download.html (can't confirm this link 100%, on my Mac atm)
Table editor: d2mods.info/filecenter/dload.php … le_id=1054
List/data file: d2mods.info/filecenter/dload.php … le_id=1540
1) Download all programs, take a backup of your patch file
2) Open up Ladik's MPQ editor, select your patch file and list file. This will give you a list of all the files available inside the patch
3) Once file is opened, navigate to: data/global/excel
4) Look for files: tressureclass and tressureclassex
5) Open these in the table editor
6) Look for the column named 'NoDrop'
7) Change all numbers in this column to 0
8) Copy the whole data folder to your LOCAL Diablo II (not on the server, somewhere you can boot D2)
9) Find the shortcut you use to boot D2 from (mine is Diablo II.exe), right-click on the exe and go to properties
10) Amend the file path to have -direct -txt (this will bypass the patch MPQ and use the files inside the data folder)
11) Enter a game. If no errors - we're good to go!
12) Package up the MPQ again using the MPQ editor
13) Test the patch file again, if no errors or anything then you can place it on your server!
Anyways, that is the basics. I'm sure you can find me on Slash's subreddit or Discord channel if you have further questions. GL!