If having global state on a single server is important, you get to choose two:

1. Long term storage
2. Censorship resistant
3. Cheap

A few of things:

1. You can keep paying for storage and as long as someone is running a valid Hub your content will be preserved. You could even make a smart contract that drips out payments for 100 years.

2. Anyone can pay for someone else's storage ($3 in OP ETH)

3. Back up off protocol on Arweave or onchain

If I've been on Farcaster for over a year but haven't come close to hitting my storage limit (~900 casts out of 10k) do my casts older >1 year still get deleted?

If you don't pay in October for storage when your 1 free year expires for being a beta user, yes.