The RATP has announced that it has resumed (as of today) the physical sale of tickets in cash from the bus driver.
Of course you might not want to rely on this for all your bus ticket needs, as the tickets cost €2 when bought from the driver versus €1.90 when bought from a vending machine or on a Navigo Easy card (or a bit less, if you use your Easy to buy a carnet of 10 tickets).
Nevertheless, it is a handy thing in case of need - I remember last summer being over at the Champs de Mars and running into two visiting families who had planned to buy tickets directly from the bus driver - but the company had suspended that possibility due to Covid restrictions.
So now, if you want to catch a bus and don't have a ticket and aren't near an RATP ticket machine, you have an option if you have a couple of euros on you.
(Yes you can buy one via SMS if you have a French cell phone number, but I figure that doesn't apply to too many people here.)