Rotom is a static encounter at the Old Chateau, specifically on the second floor in the second room. At night, interact with the TV, and you'll engage in a battle with a Rotom at Level 15. This battle only happens once however, so make sure you catch Rotom, and be careful with knocking it out.
The reason behind Rotom not being listed as obtainable in BDSP aside from transferring would be because the site lists Pokemon with static encounters in BDSP as being unobtainable. As a result, the cover art legendaries are also listed as being transfer exclusive in their own games!
Note: Rotom is only available once you unlock the National Dex, meaning that you unfortunately can't use it in your BDSP playthrough, unless you can trade for it.
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