The main developer of Robot Framework and few companies using RF heavily understood that something had to be done, so Robot Framework Foundation(2) was formed. RF Foundation has membership fees, it arranges RF conferences etc. which allows RF Foundation to pay for the development of RF.
I think that is a really good way to fund OSS development. Those companies which benefit the most from it, pay membership fees and gets to vote on the direction of the product.
(1) https://robotframework.org/ (2) https://robotframework.org/foundation/