[wizzwizz4]

Obtaining the exhaustive list of instances is very difficult, but typing your old handle into the search box of the ten largest Mastodon instances will get you many leads. Aiming for an exhaustive list is somewhat security theatre: chances are, many of your Fedi posts are in Common Crawl, which the instance sending DELETE wouldn't affect – and good luck redacting Common Crawl. Fedi is not a secret medium. (This is not to say that the maximalist "if it's technically possible, I have a right to do it!" position is anything other than a huge violation of privacy.)