Just wondering if that was something special they did for the end of S2, or if that is how it will always be now?
I kind of liked the change... I had stopped playing ranked in S2 because I didn't want to get on an unlucky streak, but if it's only your highest that matters it encourages people to play a lot more.
Season 3 will work differently in many ways. It won't exactly use highest rating, but once the season starts there will be something similar that ensures that playing more can only help you earn better rewards.
Edit: Note that the medals that show on your profile right now (if you've finished your placement matches) are not necessarily indicative of rewards that you've qualified for in Season 3. The preseason is still showing the Season 2 requirements. There are big changes coming and you'll have to wait until Season 3 actually starts to see what tier you're in with the new ruleset. That said, attaining a higher Elo during the preseason will of course count towards your standings in Season 3.