It's a pretty bad wall for NatDex AG, on the whole. It receives stiff competition from Necrozma-DM as a defensive Steel-type. It's a huge momentum drain versus Pokémon like Eternatus which doesn't mind Leech Seed's passive damage. It can't hurt most Pokémon without Leech Seed + hazards, and it doesn't have room for moves like Toxic. It also lost its pre-DLC appeal when Celesteela and Skarmory were used as checks for Pokémon such as Arceus-Ground and Necrozma-DM. Skarmory had more utility as it had access to Toxic and Spikes, but both of them fell off soon as they couldn’t adapt to the new post-DLC environment.
Celesteela’s only viability before DLC was to effectively check every Mega Rayquaza set along with Mega Gengar. The Celesteela + Gengar balanced core was extracted from USM AG, where Gengar was viable. But post-DLC, with the introduction of Calyrex-Shadow, Gengar fell off soon enough and the core dissolved, with Celesteela being forgotten. With the increment in defensive Pokémon like Yveltal and Arceus-Water, which can’t appreciate being with Celesteela, giving it a slot became difficult, and Necrozma-DM was anyways the best defensive Steel-type in the metagame since its beginning.
Currently, Celesteela’s only niche is that it can check all sets of Arceus-Ground, Necrozma-DM and Arceus-Normal, along with soft-checking Pokémon like Primal Groudon. Unless they Dynamax, Celesteela can wear them off with Leech Seed + Protect, and the occasional Toxic (at the cost of Substitute) works fine too. The biggest issue with this set is that it’s venerable to Dynamax. Dynamax moves (partly) bypass Protect and hit for much higher damage than their normal counterparts. The user would be forced to do some aggressive pivoting between the defensive core, which is bad enough as it is.
Its current rank in the Viability Rankings also shows how bad it is, in practice. In recent tournaments, Celesteela saw 0% usage, and on the ladder also it doesn’t have many supporters. Generally, a Pokémon’s usage is roughly proportional to its viability, Celesteela’s drooping numbers clearly show how bad it's doing at the moment.
If you want a defensive Steel-type, Necrozma-DM is your best bet. Unlike Celesteela, it can check the Pokémon which it is supposed to, especially Zacian. It can also function as a stallbreaker with its defensive Dragon Dance set, which can rip apart unprepared teams, using Pokémon like Arceus as setup fodder. It can also set hazards and has space for utility moves in Toxic and Thunder Wave, which can cripple most offensive Pokémon (bar Rest + Coil Zygarde).
