For those who haven’t heard about it, Flattr is a site and service that makes it easy to donate to creators. As they write:
I wish Renoise had a Flattr page I could donate to. I’d love to set up a subscription that could net Renoise some dollars every month. With enough subscribers, there’s hope this could make a difference in the budget and help with the development of the next version.
You could buy another licence 
That is called extending your own license on the backstage page.
You get a discount on the current price for the future versions that you buy into.
Hint:the current price has raised with every new version as this also affects the discounted price, so if you extend a couple of versions, you circumvent a discount price based on raised price of the new version.
This has been asked for years. Not Flattr specifically but donations in general. I had my own thread thread about this in 2007. There are dozens more if you search. The answer has always been a resounding no thanks.
I would also like to be able to donate Bitcoins (when I have any.)
If you do not want them yourself you could allways use them as give away for compo prices or whatever.
There are also many plugin developers to whom I would send bitcoins (not a whole one right anymore.) but as a buy you a beer or two thanks.
While I understand the Renoise developer’s fear of accepting donations, I believe that the amount being donated through Flattr would be so minimal per user that there wouldn’t be any expectation to receive anything back for a donation. Giving $2 or so every month would just feel good for me and would be given as a small token of gratitude for what I think is an amazing product. While being able to buy more licenses certainly works, that’s a much more clear statement involving expectations and obligations for both parties. I would much rather donate smaller amounts of money for no other reason than why I would give some pocket change to a beggar or donate money to an open source project I love.
I wasn’t aware of taktik’s own perspective on these matters, so thanks to Conner_Bw for digging up this post: https://forum.renoise.com/t/new-funding-idea-for-renoise/17619
It’s a policy that is kept maintained, so there is no use attempting to convince any of them otherwise.