At this point you can consider all non-nitro instance types to be legacy: it's been years now since all new instance types have been nitro and pretty much every instance family has a nitro replacement that is both faster and cheaper than its predecessor (except in the odd case where the sizes don't line up nicely). So it's very possible they've just stopped letting legacy instance types get new features as an incentive.

However, nitro instances use their own network hardware which includes the ENA NICS and possibly the switches, where it seems like they may have pushed their virtual network stack into hardware, in which case it makes sense that they'd not bother implementing features separately for non-nitro.

Another random network feature nitro instances get is transparent end-to-end encryption for any traffic flowing between two nitro instances in a VPC. There used to also be Network Load Balancer features that only worked for nitro backends/clients.