AFAIK there's no fork planned in the foreseeable future, so also here the block and transaction format will remain unchanged.
This mean that changes https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.18.0.md also don't touch block and transaction format?As a general rule any changes that require a fork will most likely be largely announced and thus hard to miss. No mentions regarding a fork means no change in the transaction and block format.
Check the changelog for anything that may cause troubles if you rely on specifics regarding the implementation rather than the protocol:
https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.18.0.md#0180-change-log
