When referring to a previously sent text, would you say 'I text you about that' or 'I texted you about that'?

If text is used as a verb, which it is, then its past tense and past participle are texted. As in

• I texted you yesterday
• I have texted you earlier today
• I am texting you right now
• I will text you tomorrow

I think "texted" is a perfectly acceptable, if informal word. I regularly say that I "grepped" something (from 'global regular expression'). It's a neologism - a newly emerging word.

If the informality bothers you, I would say that the more formal version would "text-messaged".

text, in that sense as a verb, was not a word until they adopted it for the new technology.

Personally I adopted texted at the same time for past tense usage.