To quote Bit Kid from the Steam community forums:

• Thieves are faster, much higher starting LCK but lower other stats, 100% crit rate for backstabs, Knife weapon damage boosted, start with Knife
• Mages have much larger starting Mana pool and INT stat, Mana refilled at save points, very bad with melee weapons (0 starting STR), higher chance for mana drop from hanging lights and enemies, start with Magic Knife spell
• Warriors are bad at the arcane arts, but can swing melee weapons faster, STR increases faster than other stats

How should these factor in to the various categories? Should we not use classes?

Thief moves 15% faster, but I don't know if that applies to backdashing or slides as of yet.

More damage from Warrior str scaling might make bosses a bit faster, but going fast doesn't tend to allow for much leveling up.

Mage also seems interesting, but the 0 starting strength hurts a lot.

And a point for not using them for now: they also might be changed in the future, and as an example: the Bit Kid account has expressed interest in making the mage backdash a short range teleport.

I havent had as much time as I would like to tinker with them, so I am reserving judgement for later, but I would look to any of the present RPGs on speedrun.com. I dont think that different categories will be needed as it could just be treated as a general RPG element, like choosing stats at level up. I think this is a great topic for further discussion, so I will absolutely try and carve out some time to give all 3 of the classes a shot and see if that changes my mind on the subject. I hope you will weigh in on it again after some experimentation. Although, in the couple of times I have played since 1.033, ive left the option off, as well. Quite enough variation as is :D

Working on getting to a point where I'm comfortable enough to stick a timer to it, I'll get back to you on that. ;P

This is my first time hearing about this update but I feel it works the same as a Dark Souls run - its up to the player execution. I haven't had a chance to dabble in the classes yet (constant back to back 10 hour days at work) but I'll do some indepth testing and report back.