If that means that, essentially, all of Unicode would be open for use on all systems, then that opens up many more possibilities (for the "special custom codes").
In theory, but it's a rare font that fully supports all of Unicode. Since this site uses Literata, Noto Sans, and Courier Prime, you may want to limit yourself to a subset that is supported by all three of these fonts. Of these, Courier Prime has the most limited support for character sets. Besides ASCII, it supports only Latin Lowercase, Latin Uppercase, Numbers, Common Latin, and Punctuation. Literata and Noto Sans both support all of these, as well as others.
My concern was support on people's systems at home; I expect that there's still a small (but, admittedly, rapidly shrinking) percentage of systems that support ANSI but not Unicode.
Those would have to be very old computers. According to the Wikipedia article on UTF-8, "UTF-8 is the dominant encoding for the World Wide Web (and internet technologies), accounting for 98.2% of all web pages, 99.0% of the top 10,000 pages, and up to 100% for many languages, as of 2024." According to the same article, Windows Notepad only supports UTF-8.
In theory, but it's a rare font that fully supports all of Unicode. Since this site uses Literata, Noto Sans, and Courier Prime, you may want to limit yourself to a subset that is supported by all three of these fonts. Of these, Courier Prime has the most limited support for character sets. Besides ASCII, it supports only Latin Lowercase, Latin Uppercase, Numbers, Common Latin, and Punctuation. Literata and Noto Sans both support all of these, as well as others.
Those would have to be very old computers. According to the Wikipedia article on UTF-8, "UTF-8 is the dominant encoding for the World Wide Web (and internet technologies), accounting for 98.2% of all web pages, 99.0% of the top 10,000 pages, and up to 100% for many languages, as of 2024." According to the same article, Windows Notepad only supports UTF-8.