Your legacy wallet should still be compatible with v28.1.
But not in v29.0 once it's released, you'll have to migrate it into a descriptor wallet.

For your UTXO Set and Blocks, the versions aren't too far apart and there's no incompatible changes listed in the Release Notes.
To check it yourself: Meticulously read every release notes from v0.18.1 to v28.1, the part where it says: "How to Upgrade" where forwards/backwards compatibility is usually mentioned.
But if you can backup the whole datadir (since it's pruned anyways), do it in case something happens with during upgrade.