For gameplay/balance reasons the devs or modders might want certain ships to be less available to players who don't have commissions without first sinking the skills into salvage; it makes sense too.
But some should be unlocked at the beginning for sure, whether it's size or difficulty based.
It's so weird that your starting frigate can find an astral on dumb luck, immediately mothball it and take it home. The way that Escape Velocity/Endless Sky did it by tying recoverability odds to your fleet's crew strength vs the ships crew strength was a good approach. You had a chance of recovering a carrier even with only one frigate, the odds were just very, very low and the game presented them clearly.
in SS the only hurdle is a big unmanipulatable RNG of whether the ship is recoverable or not; that's about the only hurdle; after that it's too easy to recover/mothball. Escape Velocity had the RNG, but you could manipulate it by getting more crew.
Some sort of salvage difficult attribute for each ship could also at least require the player to have more heavy machinery/accident risk/crew to actually recover the ship. Or if it went wrong the ship could turn into a debris field and you don't get full loot.