I suppose "local-first development" is a misnomer. It's really about the syncing part. Currently we have traditional applications that write data locally and therefore don't require an internet connection to work, and we have online stuff that updates a shared state "live". The "revolutionary" part would be having both of those things. It's a surprisingly hard thing to do.
The current state of the art here is git. So we're basically talking about making git automatic and easy to use for the majority of the population. That's not something we've been "doing for decades".
I can think of so many pieces of software that does that: having a local state, having a remote state, and keeping them synchronized whenever internet is available. It's how email apps work. That's how all cloud drives work, and Dropbox is more than a decade old at this point. It's how notes apps work. Etc. etc.
Really can't see how this can be regarded as a recent idea.
I'm someone who has used and continues to use desktop applications like that. I'm old enough to know what "work offline" and an email "outbox" is. Those are not the same things.
The always-online apps bring a lot that is not possible with such software, even simple stuff like editing the same file at the same time. Try that with tools like Dropbox and you'll get conflicts that you need to resolve. It's totally possible to get offline software if you either give up on many features or accept complicated conflict resolution etc. The basic options are either immutability, like email, or conflict resolution, like git. This is about not making those sacrifices.
"Local first" is a misnomer and that misnomer does probably reflect the different experience of youngsters, but if you can get past that there is something new here that is interesting and challenging.
Many applications work like that, particularly ones made between 2000 and 2010. Operating systems have whole stacks of SDKs and libraries dedicated to that exact behavior (e.g., CloudKit in OS X).
The current state of the art here is git. So we're basically talking about making git automatic and easy to use for the majority of the population. That's not something we've been "doing for decades".