The "leader's courtesy" has traditionally applied to leaders running for a seat in the House of Commons in a riding previously occupied by the party they lead. It does not apply to seats in which a new leader is running that was previously represented by a different party.

If the Greens are expecting a "leader's courtesy" for their new leader Annamie Paul, it would be a reasonable request if one of their three MPs resigned to make way for Paul.

Requesting it from the Liberals in a Liberal seat would be completely out of step with the convention. Requesting it of the NDP is another matter, since it isn't an NDP seat to begin with.

The notion of the NDP not putting up a candidate against Paul in Toronto Centre is really a political one, not one based on Canadian convention. I wrote about this back when the Greens signaled they wouldn't run against Singh in Burnaby South: https://www.cbc.ca/news/politics/grenier-singh-byelection-1.4742487

As I wrote about in that piece, the convention is very inconsistently applied and is even murkier when it comes to leaders that aren't prime ministers or parties that form the official opposition.