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Steel is real.
As others have said you cannot go wrong with a well built 80s - 90s mtb with good tubing, especially if you can find one with low mileage.
The earlier the bike the closer the geometry is to what we now call a hybrid, a bike suited to both on and off road riding.
All my bikes are from between the early 80s amnd early 90s, all steel accept one 1986 Klein, which I bought because it was so cheap, was in barely ridden condition and had a roller cam brake set up, more of a novelty than a daily rider, but fun to take out once in a while.
I have a preference for hand built rigid British steel frames with fillet brazing, Chas Roberts, Overbury's, Dave Yates, Dave Hinde - but I also like early Amercian ATBs, Fat Chance, Breezer, Ritchey and I'm still searching for the holy grail - an early Steve Potts or an early Brodie (Canadian).
I love their versatility, a couple of minor changes to set up and their ride characteristics can be radically altered, even just switching out the tyres to slicks and popping on a shorter stem turns a race bred trail bike into a fun street rider, but old ATBs in particular, due to their closeness to road geometry are great for touring, bikepacking, commuting, shopping etc with barely any major part changes required.
Ultimately though it's all about the feeling you get from a well made steel frame, especially one built from the best steel for the job by someone who really understood how to exploit the materials qualities.
Picrel cost me £100 and is still waiting a complete restoration.