There is currently no limitation on regular
Blue Iris updates within the major version you own, and hopefully there never will be.
Basically, at this point, you are supposed to buy one of those service plans if you need to email Ken for help with anything. Priority support is supposed to have faster response time, though I can't say if it works out this way or not.
If you don't do anything, and just let your support time remain expired, nothing bad will happen, Blue Iris will remain fully functional, and you'll still be able to download and install updates as normal.
Some people buy a support plan just to help support the developer, because they feel Blue Iris is worth a lot more than they paid for it