With UTF-7 being a requirement for certain (outdated) email protocols and programs intentionally sabotaging their UTF-7 code paths (most importantly .NET 5), UTF-7 will remain relevant for anyone dealing with email software for a while. UTF-7 is just one of the many ways in which email can screw you over.

SMTP can't reliably handle UTF-8 so your options quickly devolve into base64 encoding messages or falling back to UTF-7 encoding. Base64 is a lot more wasteful than the alternative, as cursed as UTF-7 may be. For full interoperability, some mail servers even require you to be able to deal with UTF-7 wrapped inside another transcoding format!