While you wear these boots, your walking speed becomes 30 feet, unless your walking speed is higher, and your speed isn't reduced if you are encumbered or wearing heavy armor.
But there are multiple ways in which a characters speed can be reduced beyond encumbrance. For instance, the Sentinel feat:
- When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn.
Another example is being grappled:
- A grappled creature's speed becomes 0, and it can't benefit from any bonus to its speed.
I highlighted the word becomes in these descriptions because, in the English language, it would indicate a one-time change. So a boy BECOMES a man. A seed BECOMES a plant.
But the Boots also say, "While" -- A continuous duration. This puts the description at odds.
A character wearing the Boots has their walking speed "become" 30 feet unless encumbered or wearing heavy armor. They are hit by an NPC with the Sentinel feat so their speed "becomes" 0. Now what?
Does the character's speed re-become 30 feet allowing them to keep moving? Or are they forced to stand still?
I seem to recall there being another magic item that uses the a phrase similar to "cannot be reduced below" but so far my search has come up empty.