It's been discussed here, but I don't think anyone has done it. The magic ingredient is Methyl Salicylate, AKA wintergreen oil. The common recipe I found is one part wintergreen to 3 parts 99% rubbing alcohol. Some dilute it down to 5:1 and it seems to work as well, but maybe slower. One guy mixed the oil with Xylene and it also worked, but the rubber swelled up a lot. He said that it goes back to normal after the solvent dries out. Most say that 3-5 days soaking will do the trick, some have tested leaving them longer and apparently it doesn't damage or over-soften stuff.
The carb to airbox boots on the R5 were super hard, the only slightly flexible part was the very end where it fits onto the airbox and it was nowhere near soft enough to put on. I mixed it stronger than most do, about 2:1, maybe 2.5:1. After about 12 hours soaking the less hardened area had some noticeable flex in it. They've now been in about 2.5 days and they seem soft enough to install, but maybe still a little stiffer than perfect. I'll probably pull them this evening, maybe tomorrow.
This was the 12 hour check. Hard to tell in the photo, but the lip I'm holding is flexed and it would barely bend with pretty strong pressure before.
New aftermarket boots are about $60, the ingredients were almost $50. It made me consider new boots, but I've been wanting to perform the experiment. Plus, all the rubber parts on the bike need love and I should have enough magic potion to do them all.