I stumbled across this answer that mentioned that, along with Kamino, Count Dooku erased Dagobah and a number of other star systems from the Jedi Archives. I understand the need to conceal Kamino, but why the others? And potentially related, was this why Yoda chose Dagobah? Maybe he knew it had been erased and the Emperor didn't know of it?
In the entry in the Wookiepedia for Dagobah, it tells us:
Hundreds of years before the Clone Wars, the Jedi Minch battled and killed a powerful Bpfasshi Dark Jedi in this unchartered planet. The Dark Jedi's energies absorbed into its surroundings, first tainting the planet with the dark side.
Also, from the same article, Dagobah was discovered and surveyed several times, and forgotten almost every time, so whether it was due to the tainting or something else, this was not a planet the Republic had kept track of and there was a good chance that the disappearance of several survey teams may have been connected to the tainting of the dark side.
I'm not clear on the other worlds that Dooku erased from the Jedi archives, but if it was tainted with the dark side and had been discovered and forgotten a number of times, and most survey missions had run into problems, then it's not a good place to be.
And that would make it not only a great spot for a secret base or retreat for the Sith, but it's also what would make it a good spot for Yoda to hide in. The energy from the dark side would hide his presence there and it would also make it appear as an undesirable place for any Jedi to hide, since the influence of the dark side would, over time, wear them down.
As for the other planets, it's not clear (at least in the Wookiepedia), but one can presume that, like Kamino and Dagobah, they met one or more of a few conditions:
- They were "backwater" worlds and would not be missed or sought out by anyone.
- They would be useful to the Sith, either because of resources, or dark side energy or other reasons.
- They might provide something useful to the Jedi.
I would think the first condition would mean few would meet the third condition, since if they were useful to the Jedi, someone would notice they were missing, which limits it to the 1st and 2nd conditions.
Also consider that this is one of several places in existing canon (TV and movies) we have seen where padawans have been take to test their mettle vs dark forces and fear (see TESB, Rebels S1, etc).
It is likely that Dooku removed a few planets from the record with significance in this area (i.e. Jedi/defeated Sith cultural relevance) as red herrings nothing more. It also occurs to me that Yoda may have chosen Dagobah because a) the records were erased, b) he had trained Jedi there before and might want to again due to the nature of the test and c) may have even trained Dooku there (hence Dooki singling it out for deletion, even unintentionally).