Is it worth to spend 90kEUR on putty, considering windows nowadays has openssh?
The answer is yes. The value of these bug bounty programs is directly tied to the amount of use the software gets (and most of these get used a ton, including Putty, regardless of alternatives).
Funding bug bounty programs kind of fail its objectives if they don't.
Regardless of whether they get updated, these are still a net benefit for new installs.
So given that pretty much every government/company probably has some incidental security exposure to Putty it is a smart investment to make sure it's bug free.