Not a fan of this change, honestly. It feels like it nerfs one of the more interesting aspects of shrooms/boxes in lieu of addressing the core issues that make them so problematic in Dom/TT. To wit, shrooms/boxes are both balanced around having a relatively small chance of being triggered on SR. But on Dom/TT the shorter game lengths, smaller map, and very frequent and predictably traveled paths and relative ease of invading 'enemy territory' give them a near 100% chance of being triggered. So, then the trap's duration, high damage, BFT burn, significant CC and extremely rapid deployment combine to make, well, Teemo.
(Oh, I forgot to expand on why out of all those I consider invisibility to be the interesting part. It's because invisibility is the only part of it that provides some counter-play, observing, remembering, and avoiding where the shrooms/boxes were placed.)
Will this new Lightbringer effect curb some of Teemo's power? Undoubtedly, but I question to what degree and in which situations. Firstly, Teemo's shrooms are scariest when his team holds Windmill and he is given free reign to toss them anywhere, most likely in the enemy team's direct path to Windmill. With the new change, a player with Lightbringer can still easily trip a shroom walking into brush or taking a corner too quickly; but assuming they don't and are cautious, in the best case scenario for the player shrooms can still be easily placed to force the player to stop and autoattack, avoiding the damage but likely slowing down the player's charge more than the shroom's slow would and alerting the enemy. When the player's team holds the Windmill, they will have more leeway to hunt and dispatch shrooms, but this is also the time period when the shrooms are generally more ineffective already. Mid fight, Lightbringer is apt to bring little change since most shrooms can be observed being placed or are revealed by the tower and therefore their trigger rate will mostly stay the same.
So, will Lightbringer be enough to dethrone Teemo's status of God of Dominion? Possible, mostly because Kassadin has been achingly close to usurping him for a while anyways. But I'm skeptical it'll bring Teemo to a real "in-line" state. Most good its likely to do is psychologically demotivate players from picking him every game, which I guess is an improvement, if not a true fix.