Given that 4rt asked about "6.4" and not "64", I think it's safe to say that the strings "89" and "6.4", which make perfect sense in Chinese, were also censored and that that's what he meant to ask about.

You can use "89" to refer to the year in written english, too, although I guess (a) it's a little less common since the Y2K bug hype; and (b) it's more properly written "'89", with a leading apostrophe.

Also censored: 64, 6.2+2, 6.5-1, 7.5, 6四, 六.四