Actually Bouchard's ELC is eligible to slide another season if he is in the AHL and does not get 10 NHL games next season. While the season will count as a professional season and could impact his expansion status, it would essentially be a 'free season' on his ELC. In essence, it's a 4-year ELC.
It's not common for CHL players, but it happens when the conditions are met. Bouchard is currently in his 4th OHL season so he 'graduates' out next season, he is a '99 born so he will be 20 next season before the new year, signed his ELC when he was 18 so eligible to slide twice.
Ryan Pulock was in the same situation. Same with Mantha and I think Shinkaruk in recent years.
Vegas expansion required Pulock to be protected despite still having 2 ELC seasons left, he had 2 professional seasons already since the AHL does count. I think Bouchard would be exempt for Seattle expansion so that isn't an issue here, but that's assuming the rules are the same this time around.