tyvek will fix most problems and is a lot lighter and smaller than a tire. I have never figured out how to carry duct tape, but that works too. One rider I know had a sidewall blowout, and nothing was working. He had 650b, so no chance of finding a spare. I told him about duct tape, and he managed to find some from a homeowner along the route. He rode the final 100-ish miles of a 600k on that duct tape.
The year I rode my gravel bike on brevets, I didn't have a spare tire or any way of fixing a tire other than the tried and true dollar bill boot method. So it's a legit choice in my book, particularly if you change your tires out regularly.