It completely depends on the dog breed, what it's used to, their fitness level, the activity level on the walk, where you take them in terms of shade and water both for drinking and bathing, etc. it's just about using common sense but also monitoring them carefully, e.g. if they go into a full pant instead of a half pant then the level of exertion is too much.
We have an extremely active and fit short haired collie, and we've been in Spain for the past month in a van. He's been on 30 mile walks in the mountains for 9 hours and it's been consistently over 30 degrees. He's sometimes been tired and hot, but essentially not TOO hot. Just like we were sometimes tired and hot because we are active people. We were very careful to chose high elevations with lakes, woodlands and flowing rivers, and took routes with additional shade. We also took water and had regular breaks. We monitored him very carefully and paced his activity level.
I use the golden rule that if he starts a full pant then we stop and rest and cool down.
He's been on two walks a day since we got back, at relatively normal times, in normal places. But again I have balanced his activity levels and pace with the heat, and choose the right locations like woodland with water, he is no hotter than if he is fully active in colder weather.
If I had a different breed that was less able to regulate its temperature or I was only able to do street walk with my boy then I would behave differently. I would not take a pug or an elderly dog out in this weather.
Everyone foaming at the mouth about all dogs at anything over 20 degrees isn't considering that dogs absolutely can 'live' in hot weather as long as their owners are sensible and careful.