https://oulipo.social/ is a custom Mastodon that forbids that fifth alphanum symbol in its toots. That is, using a,b,c,d is ok and any of f,g,h up to z is also ok and also digits and punctuation but that’s all.
Yup, the software is patched to not allow e in usernames or posts (apart from in URLs). Expressing yourself without using any word involving e is a fun challenge, and often you think you succeeded until the validation tells you off.