As per my understanding from the FAQ’s on https://www.hangfire.io/pricing:
If I purchase Hangfire pro subscription and do not renew the subscription, than I can continue to use the pro version, without any issues. However, in case there is any bug in pro version and our subscription has expired and I have not renewed it, will I get the fix for that issue or I need to renew subscription to get the fix for the bug?
If bug fix is released before subscription end date, but you somehow missed the upgrade, we’ll send you updated binaries. But if bug was fixed in a version that’s released after subscription end date, then renewal is required.
Thank you @odinserj. Just to confirm, in case I need technical support after expiration, I can renew it any time to get the help?
I’ve seen a lot of products that contain a language that technical support will be available some days later after subscription is purchased to avoid cases when subscription is purchased as soon as there are problems and not purchased when everything is fine. But there are no such limitations with Hangfire, at least yet 
Ok, and the Pro and Ace packages are all compatible with latest .Net framework 4.7.2?
Yes, everything is supported since .NET Framework 4.5, and nothing will be dropped in the near future.