I tried prot spec early leveling in vanilla classic. Crit/parry are great, but I found that holding threat was damage based at level 25. If you put too much mitigation on(a shield), you don't do enough damage to hold threat, because you don't have the higher tier set of talents to use, along with the lack of defensive itemization.
Sad to say, bears will be the meta tank for level 25. Their mitigation is baked in and can do more damage than a prot warrior in prot gear. There's just not enough talents to get you to a proper "meta" prot warrior at 25. Your tanking will be with a 2-handed weapon, and perhaps a shield for bosses.
Until warriors get Shield Slam(and/or perhaps Devastate rune), you won't be using a prot spec to tank well. You certainly can tank, but it can be unpleasant without 5/5 crit talents, which makes getting the higher tier prot tree talents more difficult. I will admit that Improved Bloodrage is surprisingly good at low levels. It gives you more ways to generate rage, which at low levels and with too much mitigation, you'll never have rage.
EDIT - sorry, added more, you got my theorycrafting juices going!
If you forced me to go prot at level 25, I'd probably choose this:
https://www.wowhead.com/classic/tale...--502300005001
I can see an argument for losing the point in Improved Taunt for Improved Revenge, but only if you're employing a tanking strategy that pulls more mobs than normal onto you, which expects you to have prot gear. Unless they re-do early game itemization, I don't see there being any good prot gear to be worth it, besides a shield. You'll be getting mostly STR/STAM at that level traditionally, which all warrior specs will find useful anyhow. The 5 points in cruelty outweigh the prot benefits of increased threat. Not to mention, this build is
atrocious for soloing.
That's not to mention our lack of AOE abilities in defensive stance. Best we get is cleave, and it's 20 rage, which is *rough* to find in a defensive stance(you'll have thunder clap, but it's battle stance only, and without Tactical Mastery it becomes useless). You'll be busy saving up auto-hits to bank the 20 rage to use later, and then a mob peels off you from healer aggro. It's brutal that early, trust me. The only thing saving this build at level 25 is a total re-haul of itemization in classic.