(my opinion: not official) it must be reasonably possible to reach and interact with an advertised plinth using only in-game data/knowledge.
obvious cases: plinth buried behind a wall so that you absolutely cannot interact with it, or miles high in the sky with no way of possibly reaching it, or you can reach it, but only if you first build a 200 block-high tower on the edge of the plot so that you can then sprint, jump, and “just” manage to grapple it to interact with it, but only if you have an extra-long forged gem grapple <- obvious cases for being blacklisted
still obvious cases: plinth that is locked so only your friends or guild can access it, should not be publicly advertised as is misleading to anyone not on your friends list or in your guild; this is a public advertisement, if you want to advertise to your friends or guild members, do it elsewhere.
less obvious cases: plinth that is buried in an underground lair reachable only by a portal from another world or distant location <-- is it reasonable to find this plinth? I don’t think so… it requires the player to have knowledge of where they need to go (not described, not something they can track on the compass, not something they can do without other extra knowledge not provided in-game) to be able then get to the portal that goes to your plinth. <-- to me, very much a case of “should not be advertised publicly”. Even if you could “tag” your plinth and say “in so-and-so mall” it wouldnt help anyone that doesnt know where that mall is; its not like their is a “malls directory” in-game that they would be able to use to find it.
not trivial, but reasonable to me case: plinths in an underground lair, or high up in the sky reachable via a long staircase (advertised with a nice sign or otherwise kinda obvious as a way to get up to the sky/down into the lair) or in a big castle with a front gate that is nearby that you can explore a little and find it easily to then get to the plinth. <-- fine. if your castle is 1000m long and the entrance (only one) is on the other side… not reasonable.
still reasonable: no foot access, but a clearly visible portal leading up to your base etc from the ground that gets you access to the plinths if the only knowledge you have is the location on the compass from the shopping tabs. <-- fine. eg above your lair (almost directly) is a nice gated portal with a sign saying “shop here for blinths below!” then I would say its reaosnable to expect someone without any knowledge to guess they can use that portal to get to the plinths below that is being tracked on the compass.