I would answer this in a similar manner to my answer to this question, however in this case I would ultimately say
It is up to the discretion of the DM
Warding bond has a very specific material component (emphasis mine):
a pair of platinum rings worth at least 50 gp each, which you and the target must wear for the duration
As I contend in the linked question, warding bond has a direct link between the wearing of the platinum ring and the duration of the spell, so the spell immediately ends if the ring is no longer worn. So the question becomes:
Is the polymorphed creature still considered to be wearing the platinum ring?
Regarding worn equipment, the polymorph spell uses the following phrasing:
The target’s gear melds into the new form. The creature can’t activate, use, wield, or otherwise benefit from any of its equipment.
In this case, the target is not receiving any benefit from the ring itself; the target receives benefit from a spell, the ring is merely a material component. The ring, as with the rest of the target's equipment, melds into the new bestial form. At this point I do not believe there are any hard rules governing whether a melded item can be considered "worn". I would point out, however, the following text from the druid's Wild Shape class feature does in fact make a distinction between merged items and worn items (emphasis mine):
You choose whether your equipment falls to the ground in your space, merges into your new form, or is worn by it. Worn equipment functions as normal, but the DM decides whether it is practical for the new form to wear a piece of equipment, based on the creature’s shape and size. Your equipment doesn’t change size or shape to match the new form, and any equipment that the new form can’t wear must either fall to the ground or merge with it. Equipment that merges with the form has no effect until you leave the form.
Because Wild Shape gives you the option to wear an item (if the form will allow it), while polymorph does not, I (personally) would rule that the subject of the polymorph spell is not considered to be wearing the ring after their transformation, and thus the spell would cease as well. But again, this is not a hard rule, so your DM may decide that a platinum ring melded into the appropriate appendage of the bestial form is enough to satisfy the requirements of the warding bond spell.