Is the Condo Association responsible for maintaining, repairing or replacing the threshold of condo units hallway doors or not

The documents must or else there will be a legal battle over who has to replace it. Have a lawyer review the documents as he/she can find things a normal person wouldn't understand.

Unless otherwise stated, if it is common elements the association, otherwise the unit owner. The declaration defines this, consult with an attorney

Ask the Board or property manager what their position is. If they say it is your responsibility, ask them to point to the specific provision of the condo docs or other legal authority that supports their position. If they cannot point to anything, and they won't cooperate with you regarding the threshold, have an attorney threaten to sue, then if necessary, go ahead and sue if the attorney advises to do so.

You would need a lawyer to review the condo documents to provide an answer, but it's most likely cheaper to replace the threshold yourself. Most condo docs state anything from the walls in is your responsibility and from the walls out is the COA's responsibility. If the COA is responsible for the doors, then they are most likely responsible for the threshold, which is part of the door. You can ask the COA, but my experience is they will tell you anything to keep from paying for something. Suing them over a threshold is not logical and threatening to sue them over this will most likely have the wrong effect resulting in you being targeted as a trouble maker. Many of them boast 95% of their owners cannot afford a lawsuit, so they get away with a lot. I would save your energy for a bigger battle.