+/-8 meters from it's starting height.

Pretty sure there are mods that can circumvent this limit, just check the Valheim Nexus or Thunderstore...also be aware the limit is there for a reason, most likely performance tanks beyond a certain point. I noticed with one base I made that if i edited alot of terrain it tanks my fps.

its been this way for a long while, at least 10 months or so, if not always (not sure, I came in later).
you can raise up wade-level or higher ocean land up quite high. Higher is better, as the waves can be intense at times.
trying to raise the ocean floor out in the deeps is only useful for a bridge and even that is iffy.

There is a limit for raising ground. Haven't measured it exactly, but it's something like 8-12 meters from the base level.