They do. E.g. Dale Thomas had one in his contract that got a bit of publicity when there was talk that it was being jeopardised by Carlton playing him in the reserves. In the end Daisy agreed to waive it:
https://www.foxsports.com.au/afl/da...t/news-story/b07914430842b334ee4fb2f4bfd28eff
If there was no trigger clause, what benefit would the club get from signing up a player mid-year that they didn't want/intend to keep on the list? They would have been perfectly able to leave Priddis' contractual situation unchanged until the end of the season, then gently leaned him towards retiring at that point. There's no benefit in signing him up for 2018 only to retire him and pay him out, with the payout going into the salary cap.
Your version of events is nonsensical. As I read it it involves a conversation like this:
Priddis: I want to play on in 2018, you said I could. Even though it's not in my contract.
Club: OK, well we'll give you a contract for 2018 then. But we might get you to retire gracefully at the end of this year, then pay you out.
Why wouldn't the club just say instead: "Look we'll make that call at the end of the year. We can't promise anything."?