Ah, Flash… the software people love to hate from the software company people love to hate. It’s not like they didn’t earn their reputation by repeatedly having security vulnerabilities for a decade or more now.
Flash is basically a scripting tool and a set of codecs. It’s necessary for your cameras because they output video to a codec that Flash supports (and possibly one that is not well supported by HTML 5.)
The answer would be for Netgear to rewrite their website, and possibly also their camera firmware, so that everything worked in HTML 5 without Flash. This, of course, will cost significant money for them. They’d probably rather just declare those affected products as legacy and leave support of them up to the end user. They would hope you won’t be so pissed off as to not re-buy from them in the future. (This is akin to the issues with Sonos declaring their older products unsupported.)