Which Elo's Are Considered Low/High?

Low/High doesn't mean anything without context.

But as an approximate reference, the mid point of the Ranked playerbase is roughly Silver 4, with 50% below, 50% above. Gold is the top 30-35%. Plat is the top 10%, Diamond the top 1%.

mid-high diamond I consider a "high" elo.

I consider myself a pretty bad player and am often placed with random teammates ranging from unranked to plat.

Some consider plat to be high. They are better than gold for sure, but not by as much as one would want to believe.

silver = the pros

everybody else = n00blords

I consider Brz and Silver the low ellos Gold and Plat the middle ellos Diamond+ the high ellos.