The stupid simple way I did this ages ago was:

1. Start with a-z.

2. Drop all vowels, numbers, most homoglyphs, and the letter 'x'.

3. Map digits 0-9 to one of the remaining letters.

4. Stringify the integer and replace the digit in each decimal place with its corresponding character.

For my use-case, all the numbers were >7 digits long, so the odds of you getting an offensive acronym were reasonably low unless you started combining them.

But there's no perfect solution. As this dataset shows, you can find offense in almost anything if you look hard enough:

California Personalized License Plate Requests Flagged for Review 2015-2016: https://docs.google.com/spreadsheets/d/18IUVU9Q4uN_lxqNd5AsN...