In Fallout 4 it is a bad idea to leave power armor at a location where it can be accessed by other NPCs. Both in your settlements and in the open world power armor can be taken by idling NPCs or worse, hostile NPCs. This can occur regardless of whether your armor is left free-standing or attached to a power armor station.
There is a way to safely abandon power armor though. Remove the fusion core. A fusion core is required to operate a set of power armor and (almost all) NPCs do not spawn with one in their inventory. If you store your power armor here, or anywhere else, you can rest assured that it will not be stolen as long as you remove the armor's fusion core.
Edit to address comments: Some searching on the internet suggests that power armor frames do not ever depsawn, and alleged instances of it despawning are actually instances of the frame being stolen by NPCs. With that said, people seem to think the exterior cells (such as Diamond City) reset after 30 days in game time, causing dropped items to disappear. If you visit more frequently than that there will be 0 risk of any changes of any items left behind. The power armor shouldn't despawn regardless, but visiting once a month is an easy way to restart the cell reset clock and alleviate such fears.