To hopefully add some additional clarity to my response above relative to Snort package deprecation in pfSense --
A good analogy might be the current state of the legacy ISC DHCP daemon and the new Kea component. The ISC DHCP daemon is still present in pfSense and likely will remain available for quite some time in the future. But ISC has announced that Kea is their future, and it's where all future development effort from them will be concentrated going forward. pfSense has made the decision to add Kea and to eventually deprecate the legacy ISC DHCP product.
Similarly, for Snort, the upstream Talos/Cisco team has made it clear that Snort3 is where their future development efforts will concentrate. I expect the old Snort 2.9.x tree to get very limited "love" (if it gets any at all) going forward.
But as long as the 2.9.20 binary code compiles in whatever FreeBSD version pfSense is using at a given point in time, and the code runs without crashing, I suspect the Snort 2.9.x package will continue to be available on pfSense.
On pfSense there are two pieces of the Snort package puzzle. There is the GUI component the user interacts with (written in PHP), and then the binary daemon (written in C) where all the actual packet inspection happens. The binary daemon comes from the upstream Talos/Cisco folks. All the PHP code does is create the snort.conf file and then launch the binary daemon. There may be occasional updates to the PHP code (for example, this most recent one) to address known bugs within that piece. The binary piece on pfSense also contains a custom plugin I wrote that handles the Legacy Mode blocking duty. Sometimes that custom plugin may get a fix (as it did in this release), but no new Snort binary traffic inspection features or support for new protocols are going to show up unless the Talos/Cisco upstream team makes an update for the Snort 2.9.x binary tree. I don't expect that to happen often, and it is less and less likely as time progresses. Already it's been nearly two years since any change was made in Snort 2.9.x upstream.