OK, so I did a little research (as I am prone to do)...
There are dentists and responsible organization that advocate not brushing teeth for an hour after an acidic meal. But if you have an acidic meal (something sour), it can soften the enamel on the teeth, and it takes your saliva an hour to neutralize that effect. If you brush then, especially if you brush vigorously with a hard brush, you may gradually wear down the enamel.
Sugar is not acidic, nor is maltodextrin. Sugar contributes to cavities when it stays behind on teeth, generally between teeth and in crevices, and then bacteria eat the sugar... and the bacteria produce acid which eats away at the enamel.
The main ingredients in Soylent are not especially acidic, as far as foods go - nor are the main ingredients in most DIY - but I don't find the pH of "made" Soylent listed anywhere. I haven't got mine, yet, so can't even try to test. It is probably not so acidic that the advice against brushing after acidic meals would apply, so brushing is probably safe.
It's hard to find information on the actual acidity of foods, because there is a lot of pseudo-scientific "information" out there about alkaline/acidic foods which purports to identify those foods which make "your body" more acidic or alkaline, but this doesn't pertain at all to whether the stuff is acidic in your mouth. For example, protein is considered "acid-making," because proteins are made of amino acids... But protein is not acidic by itself, and does not break down into the component amino acids in the mouth - that happens when they're digested in the stomach by the powerful acids in there.
It's easier finding the acidity of a baked good made with oat flour, or even cooked oatmeal, than the find the acidity of a slurry of oat flour and water which sat overnight. It might actually be quickest to just ask - does it taste sour, or tart, at all? If not, it's not very acidic.