What is an Entry Level Slide?
If a player who is signed to an entry-level contract and is 18 or 19 years of age (as of September 15 of the signing year), does not play in a minimum of 10 NHL games (including both regular season and playoffs; AHL games do not count), their contract is considered to ‘slide’, or extend, by one year. For example, if a player signed an ELC for three seasons from 2015-16 to 2017-2018, and their contract slides, their contract is now effective from 2016-17 to 2018-19. An exception to this rule is that if the player is 19 on September 15 of the first year of their contract, and turns 20 between September 16 and December 31, their contract does not slide.
Players who sign at 18 years old are eligible to have their contracted extended for 2 seasons. This extension does not apply if the player turns 20 between September 16 and December 31 in his signing year. Signing bonuses do not slide, and are paid to the player regardless of a slide, this causes the annual average of the players contract to change, and therefore the cap hit decreases for this player.
I am by no means an expert, but found this in CBA FAQ. Since his ELC signing age was 18 and his birthday is Feb 18, 1999 (so he did not turn 20 between Sept 15 and Dec 31), his contract is eligible to slide for a second year (as long as he plays in less than 10 games). This would make him an RFA in the summer of 2022.
The difference between this situation and your McAvoy example is the ELC signing age. McAvoy signed his ELC at age 20 so he was not eligible for an ELC slide.