Not saying it is or isn't, maybe it gets bigger, maybe it doesn't. Maybe Riot simply doesn't like it, whatever. To be honest I had this on my mind before this Spellthief + Kelpto thing came out, I'm sure we've all got our own ideas on how we'd like to protect items and runes from others, after all.
Before bringing the nerf bat while deeply apologizing to supports that someone found a way to use their items so they're getting punished like so many other items, roles and champions before, can we try something out?
What if owning any of the starter support items, whether the whole line or not (probably better to, but might be interesting to have an option out) made it so any solo experience gained was counted as duo exp, through a passive tacked on the items. Essentially this adding a jungle item-like mechanic to the support role that, unlike junglers, couldn't simply have it tied to Smite.
Now we've all, as supports or fillers, had a moment where we sat alone in a lane and got some exp for a while and losing that option does suck. That being said, that limitation would open the door to items being easier to balance without breaking anything not support and might even give the opportunity of unique items that can only be bought by champions with a support item. Also, it wouldn't mean getting 0 experience when alone in a wave, simply that we get less, so I'm rather curious how, in an average game and even some extremes what we'd truly be losing.
This might not be the best idea. Might not even be necessary. Maybe Riot tested it in the past and it didn't work. I don't know, I'm just a guy with an idea.