Some magic items have prerequisites for attunement, most commonly requiring a specific race, class, or alignment. According to the rules on attunement:
A creature's attunement to an item ends if the creature no longer satisfies the prerequisites for attunement…
The polymorph spell contains the line (and true polymorph contains a similar line replacing "chosen beast" with "new form"):
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.
While both polymorph spells state that alignment is left unchanged, would items that require attunement by a specific race or class (or other prerequisites) end attunement once the target's statistics change?
From looking for other Q&As that would answer my question, this answer to the question "What all is included in a creature's game statistics?" looks like it includes both class and race as game statistics that would be replaced. The top answer to "Does a caster under the effect of a True Polymorph spell retain their class levels?" seems to indicate that the class is still present while polymorphed, though you can't use its features.
Best Answer
Rules as written, unfortunately so.
You've found an interaction that seems undeniably correct from a strict reading of the rules. For example, should a magic item require attunement by an elf, and you are polymorphed into a frog, you no longer satisfy the attunement prerequisite of being an elf. You are unattuned. But before you implement this ruling in your games, there are some things to consider.
That said, a number of arguments have been presented to the contrary in other answers that are worthy of consideration. In the proceeding sections, I note some undesirable outcomes from assuming that this is the RAW ruling, and give some general guidance for handling this question at the table.
This severely limits the utility of polymorphing yourself or your allies.
I have always assumed that you remain attuned to your magic items while polymorphed, and I am certain that the vast majority of D&D 5e games have ruled the same. Polymorph is a good spell. This ruling will make it quite costly. Every time you cast polymorph on yourself or your allies, those polymorphed characters have to spend a number of hours re-attuning to their magic items. This can be a significant cost if in a situation where you need those magic items to function. This cost can be so high, that it can turn polymorphing yourself into a last resort sort of option.
Discuss this with your tablemates and come to a collective agreement.
So what do you do? In my estimation, this ruling would add exactly zero fun to the game for me. I like polymorph. I like using, and would be pretty bummed out if I had to choose between being able to use my magic items and being able to use one of my favorite spells. This ruling would take away from the fun of the game for me.
But I am not going to pretend to speak for everyone; maybe you are into this kind of thing. Discuss this with your table, and come to a collective agreement about how polymorph is going to work. In my experience, even if only one character can cast polymorph, everyone is going to be affected by it at some point and would have to deal with the consequences of this ruling if you put it into place.