It's increasingly common. This particular one is a bit of a headscratcher since typically pay-once-for-an-update period is presented as a de facto subscription, but this also has a subscription.

In this case, you'd pay $150 for an 18-month window of updates, or $144 for two years of updates (or, if you like, $72*1.5 for the same 18-month window, assuming you're going to pay another $150 at month 19 for something interesting.)

Contrast that with Agenda, which is $35 to buy with 12 month of updates, or about $100 for a life-time of updates. The tradeoff is more straightforward as you're just deciding whether to bet on more than three years of features.

Or contrast with OmniFocus which is $150 for the major version, which typically is on a 4-5 year cicle, or $5/mo. In that case you're just betting that you'll use the current major version more than 2.5 years.

(I'm ignoring cashflow discount; you get the idea.)

(The app itself is fun and fast to use, and I'm not complaining or demanding special treatment. I'm just interested in how these things are priced.)

I've seen this a handful of times with libraries and other software. Typically, it's a year of updates, so 18 months is on the more generous side of things with this model.

It's definitely more popular on MacOS than elsewhere.

A lot of enterprise software is on a 1 year "maintenance" cycle too. In some cases, you no longer get support and you can continue to use the latest version from that year, and in other cases the license completely expires and you can't use the product at all.

I wonder what kind of pain is involved with maintaining every new feature release with bug and compatibility fixes forever*.