I think the answer here is because Intel wants you to upgrade and spend more $ on a new product. The i350 is probably still supported because they sold and marketed them for longer in the 1Gbits segment.
What does ‘support’ mean to you? Are you paying Intel an ongoing subscription to keep it working, or are you getting ‘this device is too old’ in from the ixgbe driver mainline Linux Kernel?
I don’t expect the initialisation code to regress in the ixgbe driver so that older devices aren’t usable. I have an X540 working with Debian 12 Bookworm via mainline Linux kernel and I will have to check pre-release Debian 13 Trixie. That pre-existing code shouldn’t disappear.