I think you're being unreasonably optimistic. Outlawing drones won't stop them. You'd have to outlaw general-purpose computers, and that law would still only last until the next war between great powers.
We did a pretty good job with chemical weapons. I realize that autonomous weapons have a couple orders of magnitude in specificity, range, stealth, etc. We don't have to just accept defeat.
Chemical weapons are little-used because they don't work very well, even compared to soldiers with bayonets, much less compared to the Bomb. Producing chemical weapons is very dangerous and requires large industrial facilities.
By contrast, precision-guided munitions make the Bomb obsolete (it's great at killing people but it's a comparatively crude means of coercion by comparison), and they can be produced inexpensively from widely available materials.
We should be planning how to preserve what we can from defeat.
There's also disputed allegations of chemical weapon use in Syria. I'm not sure why they've not been seen in Ukraine yet; I think we're back to "not actually that effective in open warfare".
The only people disputing the claims in Syria are idiots tbh.
As for why they aren't seeing use in Ukraine? They aren't very useful on the battlefield in most circumstances. Dan Kaszeta (a well regarded expert on the matter) has had a fair bit to say about this.
Chemical weapons are rarely used only because they are not very effective. If they were effective they would not be outlawed.
When we invent a version that will be practical to use, (e.g. small hard to notice drone capable to spray highly poisonous chemical right in enemy barracks) all the "success" in outlawing we had will instantly disappear.
