Based on extremely limited Googling, one of the cases where these codes are still used is written colloquial Cantonese, which lacks any major official support.
As someone who has attempted to learn to get back to my roots, I can definitely say the lack of a standard romanization is a huge barrier. Cantonese textbooks are not interchangeable because each series decides which romanization to use and some even take the liberty of creating their own.
The only people that could sort this out is a government.
- Guangdong won’t, because official Party policy at best discourages use of regional languages. There was a lot of fury back when they attempted to stop provincial broadcasts in Cantonese.
- Hong Kong won’t, because the Government likes to fumble around a lot these days, and because they have a more or less unofficial goal to integrate into China. Cantonese is not an official language, only “Chinese”.
- Macau won’t. They’re too busy trying to sweeten up China to let in more gambling tourists.
The other issue is that there are now some phonetic changes between the mainland and Hong Kong, the two places people would consider authoritative on the subject.
Some forms of Cantonese romanization (https://en.wikipedia.org/wiki/Hong_Kong_Government_Cantonese...) does exist, but their use is limited to mainly language study and transliteration. And apparently, there are multiple projects to create phonetic input systems for Cantonese (see the reference section of this Cantonese Wikipedia article, https://zh-yue.wikipedia.org/wiki/%E7%B2%B5%E8%AA%9E%E6%8B%B...), but all with very limited standardization and official supports.