Protect Wooden Table From Scratches

Tempered glass. S.M.

Poplar is a fairly soft wood. I agree, a slab of glass is the best if not the the only way to protect it from scratches in the long term.

What was the particular polyurethane coating you used (brand name and product); perhaps the coating wasn't hard enough? How many coats did you apply? Did you sand between coatings?

For some things I would use a tempered glass top as SM suggests - it would definitely do the job, but for a bar table, I might want a more natural look.

Cover the scratch causing culprit with felt or anything that is not scratchy.

Or make it float on magnets.

Buff the bottoms of beverage containers, thereby removing whatever sharp projections that could scratch the table. (?)

You could have used this, but it's not compatible with the existing polyurethane.

Since the polyurethane is already in place, just wax and buff it occasionally.