Is the replacement of the fence considered an eligible expense for an eligible dwelling?

You'll need to contact the Canada Revenue Agency (CRA) and double check it with them. If you are in Canada or the United States, you can call the CRA at 1-800-959-8281. If you are elsewhere you can call 613-940-8495. This page has more ways to contact the CRA: https://www.canada.ca/en/revenue-agency/corporate/contact-information/telephone-numbers.html