The actual research on boots, heat and tendons (I think one was in Australia on racehorse, the other well-known one in Japan) showed that when tendons heat up to IIRC something like 45C, thereās a certain rate of cell death that starts to happen. Tendons donāt have much of a blood supply so take longer to cool. Along with the cell death, thereās an increase in inflammatory factors in the tendon. So you have cell death and inflammation, which might not look like ādid a tendonā in the acute sense but would not be good for the tendon, especially not in repeat bouts over time.
This is why itās recommended that boots are only used during the exercise phase and removed immediately after.
Completely coincidentally, I was discussing boots today with the guy that drives my pony who had a successful career up to Prelim/Inter in eventing but is now having a go at combined driving. Iām doing ridden strength work with her with cavelletti for the next month or so and have borrowed some pony open-fronts for that. Because sheās going to be competing next year, I want to be careful with her legs. (Normally, I donāt use boots for schooling jumping.) Sterling told me that he sometimes does phase A of marathon (drivingās term for XC) with no boots and then just puts them on for the hazard section. The thinking here is you have 14k of marathon and want to keep the horseās legs cool when you can.
There was a good overview of the veterinary literature on boots in Horse & Hound a few years ago. The conclusions were very interesting, and not really in support of boots at all as they interfere with proprioception.