Recently Tweetbot 4 was released as a cross-platform update that’ll work on iPad &iPhone. Right now (at 50% off), it’s a $4.99/£3.99 app. Regardless of whether you bought the old Tweetbot recently, or at all.
Some people were pretty angry about this…
I agree with his assessment (go ahead and read the post), as Apple has created an environment for the expectation of one-time app purchases with upgrades for life. This is simply not sustainable for developers, especially small shops. With the amount of time and work that developers put in, I think asking $5 or $10 every so often for a new version quality product that runs on iPhones and iPads is more than reasonable, especially if you use it all the time. Supporting developers is how everyone can ensure that quality products continue to be made and we don’t end up with a ton of ad-supported, customer-disrespecting junk. Besides that, if you have the old version, it’s probably going to keep working for the foreseeable future (unless Twitter changes the API), so nobody is forcing you to get the new version.