Could be next update that borks saves, could be when this leaves EA, could be never.
Impossible to say, but I could load my EA launch character into his EA launch world without issue.

So far almost every update has worked like this:
- your saved game (map and character files) are fine
- explored areas are locked. Unexplored areas are updated and regenerated.

and my advice is to not explore far into the incomplete biomes (currently far north and ashlands, one is all ice, the other all fire currently) so that when those are completed you won't have explored regions that remain empty.

!The ocean is also a biome, so exposing every inch you can of it may be an aggravation to you in the future.

regardless, so far, we have not had our stuff wiped out very often. I can't recall it happening at all, but I came in after it had been out for a little while and don't know the very, very early game history.

** some people using mods have had some issues. Also a few people have had their bases mangled by terrain resets, which I am not sure if came from mods or if sometimes the explored/locked thing does not work right. So be aware that a few people have had issues for one reason or another, but most people seem to have their stuff intact across the last couple of years. Mods are not sanctioned yet because its EA, so they are basically hacks that do what they do, use at your own risk and expect breakage on update.

Finally, we just had a major update. I would not expect another for a good long time. So your work should be safe for months, more than enough time to beat all there is to do. I do expect to eventually have a release day where we may lose it all and have to restart. But that looks to be a couple years off.

unless a game has a specific way of updating its game word after updates,
then yes, expect that big updates can indeed break current saves.
good example, Stellaris. every major update has broken past saves.

Played since, well, since the beginning. Usually I take part in public test when they are available, like now. There was one patch that caused the game to crash at random interval but other than that it has been smooth sailing.
Started great many worlds and some characters and they all seem to work to this day.

Hopefully this answers OPs biased question.

Also, characters and maps are saved separately. If your map becomes outdated, you can move the character and their stuff by logging out of one world and into another. This makes it hard to keep players on a server from bringing in outside stuff, but makes it easier to pack up and move.

I've not experienced a borked save yet, around 500 hours in. I've recently started a second world, but it was my choice and the old world still is fine. No mods. No guarantees, but no issues for me at least.

I bought it on the first of July 2022, have been playing more or less often, one seed and one character only so far, had not a save get messed up yet. After Mistlands update the game took considerably longer to load my world the first time around, that got me worried a tiny bit, but after that it worked like always again.

Looks like a lot of people are using mods. Maybe using those increases the chance of your saves going bad. I don't use any.

But this is EA, and they might change up a lot of things, so the fear of my saves not working anymore is a latent worry I have.

The major thing that makes saves go bad is steam cloud. Turn that off if you don't need it.