If i had to guess, what they're most likely refering to isn't onomatopoeia, but it's pronunciation. Vore in Japanese is 「丸呑み」(swallow whole) which is pronounced as まるのみ and sounds like 「丸のみ」(only circles) .
Using this as a rule for the date, the digits must be composed of only circles, i.e. either 0 or 8. This leaves only ８月８日 which fits the criteria.
FlorisBoard kinda died off for a year as patrickgold tried to implement statistical NLP suggestions. Thankfully, quite recently he opened up developments for contributors and people looking to help with the project on a long-term basis.
The alpha version of FlorisBoard has preliminary support for shape based Chinese layouts so you can write 汉字／漢字 with say, Cangjie or Zhengma. It also has the JIS kana layout for Japanese. Although, it doesn't support Kanji Substitution yet, but you could use the shape based Chinese layouts for that in the meanwhile. As for Korean, it's supported but no Hanja support yet.
While there isn't a release for it yet, you can take any release artifact from the GitHub Actions CI and it should have it.
Hatsune Miku: Project DIVA series