The only allowed escape sequences are "\*", "\?" and "\\" (without quotes) to produce a literal star, question mark and backslash, respectively. Without the escaping backslash, the star and question mark take the role of globbing operators similar to shell globs which is why they have to be escaped. No other escapable characters than "*", "?" and "\" exist.
Refer to https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/ for details.