Is the language used in Chinese historical dramas usually antiquated/extra formal, and does that make them a bad (or maybe just too advanced) resource for practicing one's Mandarin? (I've just closed off HSK3, for reference.) There are a number of dramas I'd like to watch (mainly "The Untamed / 陈情令" and "Oh My General / 将军在上"), but I'm wondering if they'll get me used to out-dated language and cause me to sound like I'm 500 years old :^)
You can just learn the pronunciation in historical drama. There are no doubt that the language you learn from historical drama must be out-dated language.Such as “盘缠” means money, but we all use “钱” now.
If you want learn language from TV play, i think you can watch urban drama instead of historical dram.
Your worry is reasonable. Historical/fantasy TV series of China do use deprecated or ancient expressions which may not be suitable in modern daily use.
I'd rather recommend documentaries or casual talk shows in CCTV-4 with dual subtitle, which use standard modern Chinese spoken by middle-class Chinese people.