Is this a subscription service?

It's a 1 time fee, not a subscription. And for that 1 time fee, you get the most current Census demographics available at this time. 
Do I get updates as new data are released? 
Next year's data isn't included in the price of this year's dataset, and you could choose to buy next year's data when it's available. 
For small geographies like zips/ZCTAs, the US Census Bureau collects surveys for 5 years and then sums up the responses to produce current year estimates. For the 2019 dataset, surveys were collected in   2019, 2018, 2017, 2016 and 2015. Then, the later year survey responses were projected forward (e.g. 2015 income was projected into 2019 dollars). 
The 2018 dataset is made up of surveys that were collected in 2018, 2017, 2016, 2015 and 2014. There are 4 years of overlapping survey results between the 2019 dataset and the 2018 datasets.   What this means to you is that there won't be a lot of change between 2019 data and 2018 data. Most of our clients wait 2 to 3 years before buying a more current dataset.

The only clients who get year over year updates are those clients, who for marketing purposes, have to be able to say "we are using the most current US Census data." So marketing drives the data update decision rather than changes in statistics for these clients.