I am playing a world I created in 1.14.4, which has now been updated to 1.15. The wiki states natural bee nest creation to occur only upon world creation. Does that mean bees will not be present in migrated worlds, not even in unexplored areas? I'm willing to explore to find a bee nest to bring back to my base, but if it's a no go I'll just /give myself a spawn egg.
The game only generates new chunks as you explore them, so new features added in updates will be present in unexplored areas, regardless of how old the world was (unless you used a tool like this to pregenerate the world, and even then it is only practical to generate so large of an area, so this will only ever be an issue in limited-size worlds, like a multiplayer server which pregenerated a world with a limited world size).
This is also mentioned in the Wiki, if not so clearly (they say "world generation", not "world creation", and "world generation" nearly always refers to the generation of new chunks). As far as I know, there is only one instance where new features would not be present when updating (this is the only time Mojang provided backwards compatibility with older worlds so they wouldn't have abrupt terrain changes, which would certainly not be an issue with "decoration" like beehives).