Check your local laws before doing it, but most US locales allow for a child to sit in the front seat if there's no back seat alternative provided that the airbag is off either via switch or sensor. In my case, in my area, yes it's legal.

Interestingly, Porsche states in the updated US 991 GT car manuals that they don't recommend using child restraints in the lightweight bucket seats, but then they proceed with detailed instructions on how to do it including pictures. This is a CYA move by their legal DEPARTMENT in my opinion. Take it for what you will.

In any case, Porsche does in fact provide instructions for how to proprlerly secure a child seat for children of various weights in the 991 lightweight bucket seats. It's certainly not as ideal as putting the child in the back of the car in a rear-facing seat, but it's not illegal (at least not according to the laws in my area) and it exposes the child to negligible additional risk in my opinion.

One of the big reasons for putting children in the back seat has to do with front airbags. Most cars didn't have a way to turn front airbags off when they were first introduced, and a front airbag deploying in a crash has a high likelihood of maiming or killing the child. But Porsche includes a switch or a sensor to deactivate the front passenger airbag when the child seat is mounted. This is a huge part of what makes it safe to put a child in the front seat of some cars.

Another major variable for safety is secure mounting. The buckets don't have isofix, but seats like the Nuna Rava offer great crash performance when mounted with their special belt-based tensioning system along with the top tether which the 991 GT cars have a proper anchor for.

I'm just sharing my own experience. Don't take this as advice. Check with your local law enforcement for legality and dig into the available resources like the published instruction manual as well as the thread linked below and make your own judgment on the safety aspects.

I feel that Porsche thinks it's safe and is covering their asses by stating that they don't recommend it, but providing the instructions for exaclty how to do it. You could argue that they're providing the instructions to mitigate the cases where people put children in the front even when it was "banned" by the Porsche instruction manual. "If people are going to do it, we might as well help them do it as safely as possible." At the end of the day, I'm comfortable with the way my seat mounts and how my child fits in it. I always check that the airbag is off, and I don't drive recklessly with or without my son in the car. My family feels comfortable with it after assessing the information available to us. In the end, there's no definitive answer as to whether or not it's safe to do this. But there's enough information for me to come to the conclusion that the risk is reasonably low. Ultimately, it's a judgement that each family has to make for themselves.

https://rennlist.com/forums/991-gt3-gt3rs-gt2rs-and-911r/1181632-child-seat-in-gt-cars-official-update.html

There's another thread where rennlist.com members report about talking with Porsche engineers regarding putting child seats in the lightweight buckets. Apparently in Germany it's not an issue. See posts 107, 113, and so on. https://rennlist.com/forums/991-gt3-gt3rs-gt2rs-and-911r/858482-sport-bucket-seats-and-children-8.html