I am not sure I understand the question, but in 2e the 20th level limit doesn't seem like a hard limit, as you can keep spending the thief skill points until you reach 95% in an ability. I haven't done the calculation, but it seems unlikely that by 20th level you will have capped all thief abilities at 95%. As for the elf thief level limit, the 2e DMG has rules to exceed all limits by paying an XP premium, which may "hit" after you reach the maximum level adjusted by prime requisite (but ask the DM about how s/he rules on it.)
I was saying nothing about abilities so am not sure where you got that information from. I am only talking about level limitations for demi-humans. In this case for the Elven racial class as a Thief. I started playing D&D back in the late '70s and continued on and off in the '80s and early '90s. I have the Basic and Expert Box Sets, 1st ed, rules with the PHB and D&D Rules Cyclopedia and later pn bought 2nd ed. PHB and DMG and other books from that.
Let me try again with the question as I am not running or playing in a campaign at this time. I noticed this problem while running through my Castle Construction in the Forgotten Realms.
This is in accordance with the 2nd Ed. DMG pg. 15 with its optional max level limits. I can only go to certain levels for demi-humans. I have 2 Elven Thieves that are 17th and 20th Level, respectively, the 17th Level Thief is a Female High Elf that has a 17 Dex, max limit for a Female, and a 20th Level Male High Elf with Max 18 Dex ability. According to the 2nd Ed DMG and its optional rules I am only allowed to go to 14th and 15th level as a Elven Thief with the max Dexes respectively. Since I noticed this now and want to run a campaign using these chars am I allowed to run back the XP and multi-class the chars? Or should I go on with the Levels and XP, per the 0D&D Rules Cyclopedia, pgs 24-26, for an Elf where there is NO Level Limit for races?
I do have 2 Fighter/Magic-User High Elves that started from 0D&D as well that are progressing past there level limits but have not exceeded the 2nd ed. rules yet. All my mid-level chars (12 of them in total) started from the Basic box set from 0 Level back in the '70s. I have 2 Dwarven Fighters as well.
I am considering running the XP back for the Elven Thieves, 1 being either a Thief/Thief Acrobat or a Thief/Magic-User and the other being a Thief/Ranger if this is possible? I do understand the balancing issue with 1000 year old Elves and 100s of year old Demi-Humans issue for the game.
Thank you in advance for any suggestions!