‘Encumbered’ isn’t a special game condition, you’re encumbered when you’re carrying heavy stuff, same as in real life. That sort of thing can be negotiated at the table (or you can make a rule for it yourself), this text doesn’t address it because I prefer not to use rules for that sort of thing in my own games.
The main reason to take plate armour is that it looks cooler. I think it’d make sense to let it take multiple hits, or split it into discrete pieces of armour that can each break. Again, that’s for the table to negotiate.