I own Forsaken and Season 8 pass. If I buy Shadowkeep now, do I essentially waste a season pass?

Or will I get Season 9 when they arrived?

this. provided you log in during season 9

you will get credited for the next season.

I bought SK for both xbox and ps4 and was credited the next season.

You will be refunded 1000 silver the moment you buy Shadowkeep.

This is precisely enough silver to buy the Season 9 pass (assuming you don’t spend it until then).