While renewal is important to get the best use of the system, there is a difference in the way Fortinet handles it versus some other vendors. The short story is that you are effectively paying for definition updates on the majority of services, not necessarily the ability to use them.
The long answer is that anything that can operate fully on the box will continue to operate even after your contract expires, just with whatever the last definitions that were downloaded. This includes basic web filtering (everything except fortiguard categories), IPS, and AV. There are also functions that don't require any update license to use such as DLP, VPN, SD-WAN, Load Balancing, SSO/Authentication, and Application Control (it was made free at some point). So if you don't renew your fortiguard services, the firewall will not suddenly stop working. I believe on SonicWall and some others, as soon as your license expires the functions simply shut down.
I worked in an environment years ago (FortiOS 3.6-ish) that was significantly budget tight- and we were able to make it work by dropping to only firmware update/hardware support on about 100 branch office firewalls. The use case was so restricted that the web filtering was only a small url whitelist and IPS wasn't a requirement at those locations.
