i've been seeing a decent number of 
s build relic shield (even though they don't get the execution). coin is the recommended income item. why do people not build frost queen on thresh? is there a situation where it would be a good choice over the other two items? is the slow, ap, cdr, mana regen, and extra harass not as useful as the hp regen, mana regen, and movement speed boost of the coin or the shared gold of the shield?
It's bad. Very bad.
It relies on constant harass and while thresh has some decent auto attack harass, it will not mean much in most matchups. The early sustain provided by the execute on relic shield gives you a significant advantage in early game trades/hard engages which is what you pick thresh for (getting kills/summoners/forcing enemy out of lane right as you hit level 2 or level 3). Spellthief doesn't help with this at all.
Coin is a bit mediocre on thresh due to lack of direct combat stats but if you truly want to min/max then you should start relic shield and then sell it for a talisman during the mid game once the execute on shield is less over powered.
The active on frost queens claim is only marginally useful on thresh because of its range/travel time. If the target is close then you already can slow them with your flay or your ult (flay, aa, hook is usually how you'll set up your hook) and if it's a max range hook then FQQ is hard to combo with it and will usually just cause the target to dodge you that much easier.
TL: DR - It doesn't give you direct combat stats for lane (AP is weak on thresh and he's not that mana hungry given you'll be starting with 4 biscuits or 3 + mana pot every game) and doesn't do much for thresh even in the late game.
i imagine because it doesn't give you any tankiness, it could work if your adc is crushing them, also i heard that item was nerfed or something
It's definitely not optimal, but it is fun. Be aware that you'll be focusing your ap ratios over straight tank or utility. You'll have fun with it if you know what your strengths and weaknesses are with your odd build, but don't go ranked with it.
Is honestly just a bad item for any role, it cost 2200 and takes up one of the "6" item slots. Its just that really depending on your role there are far more items than it that will get the job done better. I mean seriously if you had to travel 1000 miles would you rather petal a bike or drive a motor cycle?
I always use relic shield, because its upgrades help much more in terms of tankiness. I've tried spellthief on Thresh before and it was pretty much useless if you want to build tank.
Just use it and try it out. If it works for you use it, if it doesn't then don't.
I personally use spellthief and complete frost queen on Thresh. When people say it provides no utility, I don't know what they are talking about. Isn't a slow helping? You also get more gold off of spellthief(provided you know how to harass.) I don't rely on straight tankiness or the ap ratio off of the item. The cooldown and more CC is enough for me. Even though I started spellthief I'm still tanky through Runes and Masteries. Most of the time you sell your support item at full build anyway, to buy something better.
Trust me, everyone has their own play style. Sometimes you will have to change your item build depending on who you play against.
TL:DR - Try it for yourself. Everyone plays differently.
