My guess is that they have just skipped all the new mounts, outfits - all new things pretty much.
Then they use probbly a function like player->getClientVersion() to see what protocols the player needs to play.
It is fully posible to get 8.x -> 10.x support, but you will have a super duper hard time finding someone to do it for free.
Would not even start a project like this for 500eur, you probbly don't wanna know how much I would charge for a thing like this xD
You could try it yourself, but to start with try to just accept 9.80 -> 10.79.
Start by removing mounts, new outfits, new items etc etc
Or if you wanna make it posible for players with a higher version then rewrite the protocol package for outfits and mounts, so it once again checks for the client version on each player.
Or even better, try to get 1.2 to work with 8.6.
You will qucikly realise that you can't do this since you are asking this question

No need to go deep, just to be able to login..