A donation button existed a long time ago. But it was taken off because over a few years time only some people donated. But it was in total even hardly a month's salery (old threads about this still exist I think)...
A donation button (aka PayPal) isn't an alternative to the sponsor at all. Not only will it bring far to less too live on (note to Plus!-haters out there: that doesn't mean that Patchou is a money wolf. It means he has to earn something to live on, as developping Messenger Plus! is his full time job), but also many people don't even have a creditcard, or the likes, to use a donation button.
But, if you still want to give money instead of installing the sponsor, you can always buy something from the
Plus! store.
/ 
