Well I believe the original start was to level 10
It was always 20
(Unless goblin, Worgen and Pandean was added later)
Correct. F2P started in late Cata so Pandaren weren't a thing yet. And Blizzard never let F2P play with the newest toys so it wasn't until MoP or even WoD (I forget) until they let us be Worgen/Goblin.
We were restricted to low level Vanilla areas. The mob levels were not normalized to character level so it didn't take long before you ran into an area where the mobs were too high level. Also, there was no way to get to the zones in other expansions. I think the only exception was Hyjal through clever use of game mechanics.
Professions were maxed at skill level 100 for F2P. However, the secondary professions: Fishing, Cooking, & First Aid (was a secondary profession until it got lumped in with Tailoring) could be taken to whatever the maximum was at the time (I think 375). The game would tell you that you hit the Free Trial cap, but you would still get skill-ups. And we didn't get Archaeology until quite a bit later.
Heirlooms were BiS. With a 10g limit, you had to buy them from the Honor or Justice vendors and they only sold the chestpiece, shoulder, weapon & shield/off-hand. The better heirlooms were sold by the Justice vendor. Then Blizzard removed the Justice vendor and, shortly after, it was discovered that those heirlooms were sold at the Darkmoon Faire. So then began the scramble for DMF tickets.
Being late Cata, F2P started out with the OG talent tree. Then, in MoP, Blizzard changed that to where you picked a class specialization at level 10 and at every 15 levels, you got to choose 1 of 3 extra talents. I actually quite like the current talent system; it is quite a bit like the OG talent tree system with quite a bit more freedom IMO.
F2P could not be DKs as they started out at level 55. However, there was a bug/glitch/accident that did allow some F2P to make DKs and play them. But I think that was fixed not too long after it was found out.
My favorite thing about the start of F2P is that we were considered XP-on. And all the battlegrounds were practically 100% F2P. It was amazing and so much fun. Once in a blue moon, a leveler would make their way in but they could hold their own against F2P as long as they were decent players.
And, lastly, XPOff was TwinkInfo back then.