Do cards like Mana Tithe and Gutteral Response still exist in the color pie slices in which they were printed or are counterspells exclusively a blue thing now?

Mana Tithe is within the color pie, but Guttural Response is not.

What's your opinion when a sub-team, like those working on a duel deck, for example, want to use a pie-infringing card (e.g. Guttural Response in Mind vs Might)? I imagine you're not happy, but how heated do the conversations get?

Here’s the line I’ve drawn - If it’s not adding a card to an environment that doesn’t already have it, I don’t worry about it.

Do you think we could get more counter spells like red elemental blast or guttural response: Gruul counter hate?

That’s not something red does any more. It’s not really something it’s done outside of early Magic (with the one exception).
Red can copy spells though and that can be used to counter counterspells that can counter themselves.

Is Guttural Response a color pie break? I'm surprised at how recently it was printed; it was reprinted in 2017.

We reprint color pie breaks in reprint sets if they aren’t adding the card to a format that doesn’t already have it.

You've recently said that cards like Guttural Response are color pie breaks and shouldn't be allowed. Does that also extend to Lapse of Certainty or Mana Tithe? As someone who LOVES blue counterspells, I think conditional counters have an excellent home in white.

White mostly counters proactively, but it can counter through light taxing or temporarily through delay.

Regarding countering, you didn't mention red or green. Are you not interested in cards like Guttural Response anymore?

That is off color pie. Colors aren’t supposed to use other colors’ effects against them.

Is Guttural Response okay in terms of the modern color pie? Are non-blue colors allowed to get counterspells as long as they're very specific?

Guttural Response is not okay.

Having Instant as a supertype for Creatures would have some interesting interactions with cards like Guttural Response and Petrified Wood-Kin.

In my alternate world, getting back “instants” could also allow you to get back permanents with flash - depending on whether you say “instant sorcery” or “instant”.

Would it have been better for Guttural Response to do something like, target spell cannot be countered? Both Red and Green get that at times.

Target spell (or possibly the next spell cast) cannot be countered would be fine.

So you think guttural response was a mistake...*sigh*, as if it's a crime to have counterspells in a secondary color. You keep saying "they're in white, counterspells are secondary in white"; Well Mark, typically when one says something is secondary, that means there's more than 0 cards falling under that definition. (Im)patiently waiting...

Lapse of Certainty, Dawn Charm, Rebuff the Wicked. Mana Tithe was in Planar Chaos but I think we could do it in a normal set. There’s not a lot of them, but white does dabble a little (aka greater than 0.)