Prefacing this discussion with: "the current division system is a REWARD tier and not a RATING tier". MMR is your rating and determines matchmaking and "win predictions". LP is a reward you get for winning games, and is a function of your MMR versus your current tier.
Clamping is what happened to people when they were approaching their promo games and started getting 0-2 LP a win. They've done away with that mostly. I don't know where actual clamping still occurs but I think it's between Diamond and Challenger. Maybe when approaching Diamond? But not Platinum V, I'm pretty sure of that.
"Hard" clamping only occurs from Diamond I -> Master, because the system will not allow you to hit 100 LP until you have the met the minimum MMR threshold of Master Tier.
As Thunder described, a similar phenomenon can happen when your MMR is below your current tier (eg, when you get stuck in a 0 LP Gold V for 100 games). But it's not clamping, you're just being rewarded less for winning games that are at an MMR level below your current tier.
That said, there is a "witchcraft" LP thread on Reddit that claims that somehow the entire Elo system itself folded into nothingness.
i don't completely believe in the speculation of Reddit on invisible numbers that they have no access to... but if they are correct, my guess is that Riot re-synced up the division-tier MMRs with the soft reset. Since this reset was apparently very soft and that means that Eloflation was not completely negated, which means that every tier above Silver II had a 5-10 MMR increase. All the way up to Plat IV... probably results in a 100-150 MMR increase, which is similar to being "plunged" to Gold II. It'll probably work itself out after 10-20 games.