Board Thread:Roblox help and discussion/@comment-35034256-20190208065127/@comment-35998649-20190209154059

It's actually a bunch of different reasons, including the one that OP listed.

IMO the two main reasons for numbers being filtered is:

1. for protecting your personal information so you don't just give it out to someone who may be malicious in nature.

2. people like to get around the filter by using numbers to say non devex-friendly stuff (not just numbers, a lot of different symbols and characters from different languages).