"is it safe to use my 18 month-old
sugar bricks in the late winter/early spring of 2021 "
The only thing that is going to go wrong with is a bacterial, fungal or yeast growth on the bricks. Actually I doubt if any of those are even a problem based on what I see growing in my sugar feeders in summer. I have stored sugar bricks over a year with no issues. As long as you do not see obvious stuff growing on them you can be quite comfortable feeding them to your bees in my opinion.
Dick
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