According to Wowwiki, “pre-TBC” was (spirit x 0.8) + 6 ; and “TBC” was (spirit x 0.5) + 6
Given that Classic runs on the final patch before TBC launched, what Wowwiki is referring to, is actually in the current 1.12 patch, and the 0.8 figure refers to most of vanilla.
I don’t usually trust wowwiki to be accurate but in this case, the figures add up - as 30*0.5 + 6 = 21.
Warriors just have awful health regen i am afraid!
Also have a look at only the 4th point down in this list of ‘not bugs’:
Warrior health Regeneration is working at the expected rate.
I do believe it is working correctly and I would encourage you to take up cooking, or if not, then be sure to stock up on food at the zone’s nearby Inn when leveling.
Also, if you were considering using Spirit gear, its not worth it just for the out of combat regen. The damage increase from Str/Agi gear means you spend less time being attacked by mobs because they die quicker.
Another tip from me: try to configure your personal multitasking around your health regen! For example - need to send a text, or change your music, or go to the toilet, or turn your lights on, or grab a drink etc ? Put it aside until you need to regain health and do it while you’re waiting for your health regen! 